I'm babbling again, but... the point is, find a way to derive happiness, from things that you will have forever, or to just appreciate everything that comes your way, be it good or bad. Find a perspective that doesn't depend on any one thing or one person to make you happy. YOU are the one who ultimately declares what sources of stimuli make you happy. You have to find the perspective. I've only given you a starting place. Seriously, go out in a field and think about this.
Remember, what you need to do is choose the proper sources of stimuli that will make you happy. Right now you're saying "Well I've got a good girlfriend", which translates to, "I'm designating my girlfriend to be the source of my happiness." You're giving her an obligation she may not be ready for. Or, "I make good grades", meaning, "I'm designating my academic performance as a source of happiness". Can you always make good grades? What happens when you just can't learn something? When a class really sucks? When you get sick, or for some other reason beyond your control, you miss class and your grades suffer? You're putting an obligation on yourself that you might not be able to keep up with. Hell, even saying "As long as these things are true, I can still say I have a good life," is meaning, "I'm designating all these sources as my sources of happiness, so as long as the sources I've chosen are consistent, I'll be happy." But no! That doesn't work either, because no source is consistent!
Fortunately for me, I can do this thing where I can say, "Praying to God puts me at peace." Which really is saying, "I'm designating the act of prayer as something that will make me peaceful." You know what's nice about that? I can always pray, and no one can stop me. For a lot of people, this doesn't work, because they say "praying doesn't get anything accomplished in the real world, my prayers are never answered, it seems useless," sure. But that's their perspective and what they want out of it. My perspective is, the act of praying in itself is a meditation process, and I like taking time out not to think about the world. THAT is the source of my peace. So as long as I can actually get myself away from distraction, I DO find that peace, because I put it there. Because I'm getting out of prayer exactly what I put into it. I also have the perspective (and you're welcome to copy or reject) that I will NOT make "consistent results from my sources of happiness" a source of happiness! So when the shit hits the fan, it doesn't make me sad either. Shit happens, and I've accepted it. Because I haven't put pressure on reality to perform for me consistently, by making it my source of happiness.
So your "center" that you wish to find, is wherever you want to put it. Your happiness is in whatever source you wish to designate getting it from. You are far more in control of your life and your emotions than you think you are, as long as you remember that you are choosing where your happiness comes from.
Watch "I <3 Huckabees" and "Neon Genesis" (skip to episodes 25 and 26, but realize the main character, Shinji, is a huge IW with inconfidence issues). These might help.
