I normally get drunk to cope with my boyfriend calling/video chatting his fiancé and I don’t know how I’m gonna cope when he tries to move her into our house.
You should never ever stay in a situation that is so emotionally painful that you turn to alcohol (or other numbing behaviors) to get through it. Something serious needs to change, immediately.
Please talk to your boyfriend about how you’re feeling. If there’s something specifically that you two can work on to help you manage whatever feelings are coming up, find and start that. But it just might not be healthy for you to continue dating someone who also has a fiancé. It will be better for you in the long run to leave a relationship that is driving you to “get drunk to cope.”
Please also see a therapist for help with your use of alcohol to self-soothe. There is nothing wrong with drinking, but using it as a coping strategy is dangerous both physically and psychologically. You deserve help finding better ways to get through painful feelings, and to stand up for yourself in situations that hurt rather than trying to drink yourself through them.