I do think you should tell your mother sooner rather than later. It's what is fair to your BF, as well as to your mother, and it is the right and mature thing to do especially if you would like this relationship to be long term. I understand that you love your mother in spite of the fact that it kind of sounds like she makes you miserable, but I'm here to tell you (from a lot of experience), there is a point in your life at which you have to make a choice of whether you want to keep modifying your life for someone who hurts you (even though she is your mother), or set some boundaries. I had to do that with my aunt who was like a mother to me. Unfortunately, after years of setting boundaries with her which she ignored (her way of managing conflict was to either ignore it or yell louder than you), I finally told her enough was enough and we don't have a relationship anymore.

Also depression isn't an excuse to heap negativity onto others indefinitely; I certainly didn't get away with doing that in my relationships for very long!

Just one more thing, I would suggest cultivating some stronger relationships with your female friends. Or at least, friends who are not mutual friends of you and your BF. It's good and healthy to have people outside of the relationship to lean on.