Feeling for the past

This is my first time using this. I am only 15 and this is my first relationship. I am usually a social outcast and i dont understand emotions well. My boyfriend I have been dating, (no sex but have been going on dates) for 2 years. We are very much attached to each other and we've been best of friend for longer. He's had past relationship before me. His last girlfriend Ive had problems with last year. She has no sense of personal space as in pressing her body against him and flirted a lot. He's very dense so he didn't realize she was flirting. I got angry with him then I got to a point where i realized i was taking stuff too far. I tried to drop what happened between them and was being overly nice to her. I ended up scaring her to a point where she left him alone. This year i came back to school and she got pregnant over the summer and doesn't know who the father is. Now that my boyfriend figured it out, (i won the bet we made on the first day of school about her looking pregnant) he feels directly responsible and is falling into a depression. I dont know what to do with this girl. Ive done everything to stop him from obsessing and she has said in many ways how she has no feeling for him and has said she want him to stay away manytimes. He took a long time to get over her and i feel that he still has very strong feelings for her. Ive tried everything and i even suggest taking a therapy session together but he refused. This problem is tearing me apart. He is refusing to work with me anymore. What should i do? I dont want to break up because its the only reason why i would break up with him.

I apprecate the replay. He wont get that depressed. This girl is the only real challenge hes had in his life. Hes afraid of alot in life (he made a shallow nic in his thumb and called my mom because shes a nurse and asked if he was goig to die) we both wanna protect eachother. There hasnt been a day in 2 years that i havent told him i love him or havent been there for him. My problem mostly lays with him, his connection with the girl and the fact she got preg and he feel its directly his fault. I have told him i posted this and im going to tell him what others think. He is open to the thought of outside help.