My boyfriend refuses to talk to me about anything anymore. We’re always arguing, even though we never used to. I just feel so useless, everything I do upsets or annoys him, but he is a really nice person.
It’s a good relationship, but I’m just not sure how long we’ve got left anymore (and it’s not just me wondering that).
And to top it off? I’ve been slightly losing feelings for him… and developing them for one of his good friends…
what do I do??
LemonVibe 
End the relationship and wait until you are definitely over him before starting a new relationship. He’s trying to make you do that anyway by blaming you for little things. Looks like you are not compatible.
First of all, don’t pursue a relationship with his friend until your relationship with him is over, that is a given. You need to talk to your boyfriend about your relationship, and where you both stand. If you feel you two are truly irreparable, and you feel for someone else, then you should probably break it off. If you still have feelings for your boyfriend, and him for you, you should try to work it out.
definitely break up with him cus it sounds like hes really not worth it, but don’t immediately pursue the relationship with his friend, leave it for a bit to settle down
Sounds like you already have the answer. If you’re thinking about a relationship with another guy, you’re over your current bf. Don’t let the “guilt” of ending things keep you from letting him and you move on. I wouldn’t recommend pursuing his friend, that will come organically in the future if it’s meant to be. A lot of times the friend will check you later once you’ve broken up with his friend, anyway. There are so many guys out there, just enjoy being single for a bit.