I have personally experienced and heard from others that the issues you can speak easily about with your best friends are impossible to discuss with your partner.
In a serious relationship when a discussion is required on a specific topic, I think the automatic assumption is made that the problem must be fixed right there. Sometimes before starting the discussion, put it out there that you're not looking for a solution, maybe you just need a sounding board, someone to talk to and express your feelings with and you're not expecting him to fix your problems or come up with an answer to the problem right there. Also tell him that just because your expressing your feelings doesn't indicate a failure on his part as well. I will speak honestly on my own behalf that I did that alot, I always assumed that the problem was my fault, had to fix it right there, got frustrated that I didn't have a solution that worked, took the discussion personally and felt that every point in the discussion hilighted a failure on my part.
It's easy to fall into that trap, maybe it's a guy thing, we just want to fix things instead of dwell on them.
Did you want to share more on the specifics of the current discussion/argument you're having with your bf? You don't have to but it might help.