I highly recommend you start going to Al Anon. There you will be given the tools necessary to detach from your A.
In order to detach you have to break the cycle of co-dependency and enabling. The first step is to accept that you are powerless over his alcoholism and addiction. Last night instead of having the nice evening you planned on the evening was centered around your BF getting drunk. I understand the thought process of just stopping to let him get a beer rather than get into an argument (been there) however, you were driving HIM. This entire night was you in the passenger seat being told what to do with a mind set of "accept it and shut up".
Review your post - he left to get more beer and after being gone your mind started wandering if he had possibly left to get drugs. You went down to look for him because "you don't live in a nice area" to do….what? To see if he was being attacked? Or to hope that you could stop him from going to get drugs? The night then proceeded with your very drunk A wanting sex which you did not want and you once again went with it I imagine to also avoid an argument. Now you feel bad because he insulted you.
How many times last night did you do something you didn't want to do?
Boundary setting is intregal to detaching from your A. Boundary's are set for YOU not for HIM, you cannot control him. Don't even bother trying it is an exercise in futility. You can control yourself. A boundary would be saying that you won't stop to get beer, you won't have sex when he is intoxicated. You have to dismiss the fear of an argument - you aren't saying that you can't have a beer just that he needs to make arrangements himself to obtain it (and he will).
Trying to find other outlets to enjoy would be helpful for you as well. There is nothing wrong with you deciding to leave to go to a friend's house, a coffee shop, a movie etc. when he decides to drink.
Sorry you are going through this!