Hey.... I don't know if you're Christian or not, but I am, so I'd like to give you some advice.
God has a plan for you. Everything he is doing, he is doing to prepare you for the better life he has planned. He is making you stronger. He does not make mistakes, so don't try to tell me you are one. I used to think like this all the time till God brought my childhood best friend back to me. She did leave again, but I decided I am done questioning it. Because he has a plan for everyone. He will put you through rough times, but you can come out stronger. You just have to remind yourself that you are not worthless. Everyone God creates is perfect in his eyes. Imagine being a painter, and creating a beautiful painting. Then having someone come along and point out the flaws. God is the painter, you are the painting. And when all you do is point out the flaws, you can imagine how he feels.
Just remember that you are perfect in God's eyes, and that he has a plan for a better life. He is making you stronger through all this.