I haven't read every post in this thread, but something that I've noticed as I get older -- you need to find the BALANCE. From the sound of it, there's a good chance you're going too hard at the gym (or whatever else you might do).
Let me try explain more - working out really hard takes energy - calories! But what really changes as you get older, and the more you work out, is how much energy you ALSO need for RECOVERY! When you go hard you need to also go hard on rest and nutrition or you won't feel very good. This means rest, especially sleep, and eating enough good foods.
What happens to me when I try to go too hard on both ends is I hit a wall, and start to feel very tired, sluggish, cranky. So then I start to crave junk foods that aren't good for me, which fills me up but doesn't provide much nutrition, so I don't fully recover. This cycle continues until I either catch it and fix it, or sometimes get sick!
The closer you get to that 6 pack, the smaller the spare tire gets, the harder and slower the rest will come off. This is normal. You can't compare someone like Penn Gillette because he was a big fatso... the first few hundred(!!) pounds come off super fast compared to the last ten.
So, while I am no expert and can only speak about what has and hasn't worked for me, I'll offer these pointers:
1. make it a full lifestyle change. Not a 'diet' or a 'workout routine' but just how you live your life. The best way to stick with it is to make it something you don't even think about. This is especially useful if you can build in small workouts to your life. Walk more, like for me if it's less than 2 miles and I'm not in a rush, I walk. Even if it's cold or raining. But I also effing hate the subway so it works out hah. Find things that are fun and also good for you, but don't consider them your workout, they are just fun things you do. I love to rollerblade for example, and ride a bike, and go hiking. But I still hit the gym if it's a gym day.
2. take it easy on yourself mentally. Those magazine fitness models are A - professionals (they have LOTS of time to take care of their appearance) and B - look like that for a few hours - JUST for the shoot - and there's lots of airbrushing. I've read stories about how they have to basically dehydrate themselves before a shoot to look shredded enough. Maybe it's BS, but the point stands - fitness/model photos aren't real life. It's pointless to be discouraged by not looking like that. Instead focus on the good things you're doing and the progress you've already made.
3. take it east on yourself physically. Don't overtrain, which can be easy to do. Going harder past a certain point is NOT better. I find I progress better (especially if I'm trying to LOSE weight while I'm training) if I increase frequency of training and decrease intensity. So like instead of 2 heavy days of chest, do 3 or 4 much lighter ones. It makes the workout suck less and I've seen good progress, especially when it comes to shredding (as opposed to bulking -- disclaimer, I have never had a 6 pack personally, but close... LOVE pizza too much!)
4. as someone else said, a 6 pack is mostly made in the kitchen. Eat smart, eat healthy, and eat enough GOOD food. This is by FAR the hardest part for me, and I think for most people. Phone apps like mapmyfitness can help especially at first, but as I mentioned earlier I find I do best when I don't overthink it. Instead I learn what I need to do, and then make it part of my lifestyle.
Hope this helps!