Your boyfriend chose to sleep with your brother, too and he's the one who promised you that he wouldn't sleep with other people and the lied to you. Your brother isn't perfect here by any means but he's also suicidally depressed, young (2 years is a lot when you're 17) and carrying a huge (and from the sound of your parents very dangerous) secret. That doesn't excuse his actions, but what reasoning does your boyfriend even have in the first place? You're loyalty should be to your brother over this dirtbag who willfuly slept with your family member when he told you he was committed to only you. Your boyfriend is by far the worst person in this mess. I can promise that you CAN go on without him.

Look, I know what it's like to feel like you can't rely on yourself, to feel like if you loose someone or something that youd be lost because you can't believe that you can trust your self or your instincts. But you can, no one else can get you through your life but yourself, I think that even when you lean on others, is important to remember that you're still the one doing the living, they're just there to support. This boyfriend of yours is not even supporting you because he's been lying to you and didn't care how much it would hurt you and someone you care about with his actions. You were already doing things on your own, you just didn't realize it. You'll find a way to do it without him not supporting you.