mars, you're the liberal version of me. Creepy, huh?
I've been doing what you're talking about for most of my life - pushing people away, using maladaptive methods to feel better, all of it. It's a VERY hard habit to get out of. Right now, I have a wonderful boyfriend that I keep thinking I should break up with, only to remember that I'm not unhappy with him, I'm unhappy with ME. So that would be a pretty stupid reason to break up, right? It doesn't help anyone, but it's in my head all the time. I just can't shake the feeling that I'm pretty much not worth anyone's time. It's so much easier to just push a boyfriend away than to deal with my own insecurities. Do you get those feelings a lot? I'd wager you do. Or if you don't that you shove them so far down that you barely know they're there, and that's the root of the issue... I repressed all my negative emotions toward myself for a long time, and it manifested exactly as you describe.
Getting all dissociated and disengaged from life is easy to do, and hard to fix. It's just like how it's easy to gain weight and hard to lose it - it's easy to gain a psychological burden and hard to get rid of it. When you feel worthless, it's hard to make any effort to try to improve your life. It's not worth your time, right? It's too hard. You're too tired and too bored to try to get awake and excited. And even if you could, you're not worth it. It's not worth it. It's an endless cycle. And even when you try to get out of it, it's really easy to lose motivation. It's easier to just take a drink/pill/drug of choice. Right now, I'm relying on weed to keep me functional. When I'm high, I feel so happy with my boyfriend, animated, excited, engaged, ready to get going with life; when I'm sober, I just don't feel anything at all.
I know you know about all the things you're supposed to do - eat right, exercise, take vitamins, get therapy, all of that. You know that I know it too. So the question is how do we get ourselves to do it... I can't say I've been entirely successful, but every time I try, it gets easier, and I stay in a good place for longer. I'm hoping that one of these days I might get it right for good. But there's no secret. There's no trick. You just have to get up and do it. I find that making appointments for things like that helps me the most. Won't exercise? Get a personal trainer (even if it's a friend doing it for free, get someone to force you to show up X days a week and work out!). Won't eat right? Only put healthy foods in your fridge! Won't take vitamins? Get a pill box so you can lay them all out and make it easy to remember to take them by sticking the box next to your toothbrush or coffee cup or whatever.
It's not that you can't do it. It's that it *is* REALLY hard. So for lack of a better way to put it, just do it.

It snowballs in a good way, so just start out slow and easy, and you'll get there. Just try to be, I dunno a good word... "zen" about it - remember that a lot of times it's going to feel awkward and awful and that's not a reason to get upset, it's a reason to keep moving forward.