I don't know what to believe

I just feel really pathetic at the moment, I'm so emotional about everything! And I just don't feel like I will help or can help. And as it is I'm getting frazzled. I'm trying so hard to be positive but I just don't think I can do it anymore, its just too hard. I guess I can't do it, I can't make it. I can't be an adult because I just get freaked out over being an adult, doing adult things. My fiance deserves someone who can stay positive a lot, not be so up and down with my emotions, someone who is going to pray hard out on her knees, pretty much begging God, but no I don't! I give up, I'm not strong enough!
If I could give him all my happy days and in return I got all his bad days. I'd do it because I feel I've already had a good enough life but he's had the worst, he didn't have the family I've had the life I've had. If I could and I knew God would do it, I'd ask that he'd turn back time and make it that he was born in my family and I was born in his then he would get the life he always wanted and he would treat my family better than I ever have. And then I would get what I deserve by being born to his family. I guess if he does see this, it will probably be the last straw, I know he doesn't like me saying stuff like this and I'm sure he said awhile back if I kept on thinking this it would be over. And I know I couldn't take it him ending it, because I know I love him, but maybe it would be for the best. He could find someone there in the USA, someone who is more positive than me, someone who is bubbly and outgoing, someone who really thinks of everyone else instead of just themselves. Anyway thats all I have to say.