Why does my body always put on weight?

Hello! I’m 32, 5’7”, and I have a “tempermental” weight issue. The trouble is, I eat well (rarely eat pasta, and infrequently eat whole wheat/muti-grain bread, eat veggies, rarely eat sweets, eat cheeses, meats, etc). When I work out vigerously, I drop weight unusually fast, and I love the way I look. When I stop working out very vigerously, I seem to gain it so easily. I never stop going on long walks, and remain fairly active. But now I have a knee problem, so running is ill-advised, AND I’m putting myself back through school, so time and money are problems, and inactivity is the word of the day. Diet pills don’t seem healthy, though I have tried them succesfully. I just feel that my body actually WANTS to be fat, and the only way I can stop it is by working out strenuously - which I enjoy, but sometimes can’t do. I’ve tried the South Beach Diet successfully (I also seem to drop weight unusually fast when I diet - at least I have better and faster results than people dieting with me), but that is extremely expensive (and I just flat don’t buy the replacement frozen meal gimmick. I figure, do it right, or don’t do it at all).

The problems I think I have that might be contributing: I am alone much of the time, but am a relational person. Being with people motivates me, and when I’m alone, I don’t feel the same motivation. I don’t sleep well at all. I can’t get to sleep fast enough or deep enough, and toward the morning, when I should be waking up, I’m going into deep sleep finally… but now I have to wake up, so I feel groggy most of the day unless I drink coffee… I’d say that I eat very well, but definitely not frequently enough. I just don’t feel hungry, so I sometimes only eat once or twice a day - and that’s gotta be killin’ my metabolism.

So far I’ve gained 15-20 from my previous 135, which I’m bummin’ pretty hard about… That was after I left a very emotionally abusive man, went into a depression, almost completely stopped eating, and after I started coming back around again, re-injured my knee so had to stay off of it for a couple of weeks. Now everything’s back to normal, and I’m lovin’ my life… but I got this weight to deal with… gah!

Any advise?
This is beyond frustrating! :)