What happened to the old me?!?!?

It is depression. It wil follow you for the rest of your life and you have to fight it every day, but if you want to be that happy person again you can, you just have to make it your goal "be happier". Never set the goal to "be happy" but to be happier than you were the day before, and taking everything in stride you will surprise yourself at how much better you feel in no time