Same with me! This is exactly how I've felt about the cognition thing for probably like 2-3 years and I started feeling I guess differently ever since then. It started with my memory. I couldn't figure out why days seemed to just be happening as one at a time when I used to feel like I could remember days before the one I was in. (If that made sense).
I've always been horrible at math but what I considered to be pretty good/better then most at English skills (which really do more for me then math ever could, haha)
Certain people just see the slow thinking part of me and I guess that's why I really have so few friends, but in a way I think that's a great thing because then you only make friends that you KNOW like you for who you are.
So basically I smoked for 3 years, the last 3-6 months and maybe up to a year of my smoking before I stopped I could'nt hardly think straight. I couldn't remember the last time I bought gas (to see how often I buy it), I would do things and forget I had done them or not focus and not put it into memory, and I stopped caring about everything (I think somewhat related to stressors of not knowing what I wanna do with my life. So I felt pretty stressed out and anxious that I may possibly be "friying" myself. Idk what it was but maybe cannabis is not for me,
Haha this just seems almost funny because I could write for hours about my problems but I swear they are probably nearly identical to yours.
My addictive personality sometimes gets the best of me and I think the problem with weed is that when used correctly it is just TOO GOOD to resist, even though as we all know too much of a good thing is NEVER good. It's like I finally learned that lesson, or am learning it now. My depression caused me to smoke every day I believe just because I was trying to overcompensate for whatever feeling I might be having at the time (deciding what to do with my life JUST came up for me because I've been out of high school for a while and had no concrete plans. This made me feel extremely uneasy, feelings like am I a good enough person etc etc. (I have recently come up with a theory about why I'm like this but it would take far too long to explain in detail) but also with these feelings came a overwhelming sense of guilt, near the point of stopping me from doing anything that I used to get happy about thinking of.
But stopping for a week did some amazing things in my life, as cheesy as it sounds. one of which is realizing that I can still enjoy cannabis the way I used to. My GF and I went to a pretty new, but close/trusted as can be friend's house and then after a few hours of hanging out we smoked a fat joint, which I didn't expect to smoke at all. It felt like smoking weed was meaningful again!! It made me feel happy like I used to get from weed! Part of this was my mood I'm sure but the feeling was like being released from depression just for a while. It was great!!!!!
And I worried about weed not being for me too and that stressed me even more. Don't think this way!!!!! It just needs to be used in the right context, the right bud, and probably around the right people or in a situation where you can really, REALLY think straight and realize what emotions you really wanna feel. (that's the only way I can really describe it.) and I honestly believe that. One thing I kinda decided was that from now on I'm only going to smoke when I have something worth smoking for (could be anything you really enjoy from hobbies to excercising, just whatever you like.)
But I'm really thinking its a combination of my medicines I was taking by my doc's orders, smoking too many times a day, and not exercising my brain: not enough problem solving like I could get if I had a good technical career, horseshoeing, mobile mechanic, welder etc. (They say you don't use it you lose it) so I stopped and to save money too. So Idk it really bothers me that maybe cannabis just fucks with my brain. But I hope not cuz it does help level my moods and it is very enjoyable.
This is interesting because nowadays I always talk about manual, simple jobs being what I am good at and that means I can do it fast, well, etc, and that makes me feel good! I know I'm a hard worker but some jobs that require precise thinking just make me feel like shit. I think it goes along with the memory thing too. I can't explain it too well.
But Exactly!!!!! I stopped to save money and although I could just go buy weed I think you just gotta realise that it's something that needs to be treated with respect and if you treat it that way, it will give back to you too. I mean really we are the only ones who abused it like that (although I only smoked heavily for maybe a year or so.)
Also the problem I think with both of us is that when you smoke that much weed your lungs get ruined so you stop getting high especially if you use shitty weed (anything not homegrown and smoked within 3 months to me I think now is shitty), unless of course the weed was CLEARLY good.
And when you add the lung thing to a HUGE tolerance to weed just starts "not working" for you! and it's true! it no longer got me high, just made me feel "okay." I decided this was unacceptable because I think we all know how good it CAN make you feel. 
Next time you do smoke weed, make sure it is worth smoking and is dank. Make it sometime when you feel ALREADY especially happy and RECOGNIZE what it felt to be happy like when you used to smoke and it made you feel that way. Just stop and think about everything and how good you feel. Or if you don't feel good, think about WHY you don't feel good. It really helps if you have someone to voice your thoughts to, but if you don't just say things in your head but make sure they are POSITIVE. Listen to some good music you used to love or practice drawing if you like that or just doodle on a piece of paper. Anything! just let the high take you wherever it wants to take you.
I honestly didn't know how I'd feel after that joint yesterday and it was just unreal what the difference was quitting and then having some really good bud. It almost felt like a life changing experience. I wasn't even in a very good mood before I smoked it and was even nervous around my friend (that's just who I am around other people in person.)
I know this probably looks like hippie shit to anyone else reading this, but this WORKS! Like I think depression is so much in our own (people's) heads now and a lot of that comes from unrealistic expectations, of ourselves and other people but mostly ourselves. Logic is key too, I think. Alot of this is realizing that we should be happy and that you need to get done what makes you happy in the end, or at the end of the day, no matter what it is.
I don't know why this came to mind also but before I finish this, i think another SO important thing is just TRUST YOURSELF!!! Nothing stupid is your fault, it's just what you happen to be feeling at the time. If you feel shitty, it's NOT YOUR FAULT! I think we (everyone) has to accept that before we can really be happy.
Oh and one more thing even, you GOTTA realize that the stressors in your life, again ARE NOT YOUR FAULT!!!!! THIS IS KEY!!!!!!! While some things are our fault it's better to just realize that we can't change the way you feel so the best thing to do is find a way around everything negative and only leave room for positive things in your life. And soooooo much of that is just learning about yourself and finding out how to make yourself truly happy.
I hope this helps you some more because honestly I only started feeling anywhere near like I was coming out of depression after I decided to quit a week and a half ago. And things only got REALLY better yesterday after that joint and much, much thinking. 
Peace, take care!!
