NO, we do not think you are an idiot. But I feel like I can't talk to DP about this anyway because last time it ended in arguments. OK here is your real problem, your dp and you are about to become a family and you cannot talk to him. This should be a massive red flag that things are not as they should be between you.
Assuming he is not abusive to you, get some 'Relate' counselling (marriage guidance counselling) or similar. Because not being able to talk to gether is going to lead to a break up at some point in the future or you having a shit life while he tramples on your feelings.
"If this is my life before the baby is even here how bad will it be once the baby arrives? I feel like she is taking over." Then do not let her.
Lauraw1989 "I really don't want to break up with my DP I love him soooo much and I don't want to bring a baby into the world with separated parents." Then he needs to put you and the baby first. IF he has a drink problem he needs to prioritize getting it sorted out now.
Love is good but if it is not combinned with care and consideration it will wear a bit thin when you are dealing with a new baby and a pissed up partner. You need to put yourself and baby first now.
Always have the money in your purse for a taxi or the keys to car you can drive and tell your DP before you go out, I will be leaving when I want, under my own steam, what you do is up to you. (Unless you want him to leave with you then discuss that beforehand.)
"I think I just wanted him to be fair with me and realise when he needs to look after his pregnant girlfriend."
You want him to show you care, explain that, but be ready to look after yourself too.
Being emotional, crying etc can all be part of being pregnant.
Maybe your dp has nice qualities, start a conversation with that.
EG "I love you because XYZ, BUT I need more now we are expecting a baby. I need you to protect me from your Mum! Take me home, etc etc."
Then give him 5 or 10 minutes to do what you need in every situation. "Hon, I'd like to go home now." After 10 minutes if you are not going out the door together, you go.
Or "Your Mum is giving me hell." Within 5 mins you need him to be sticking up for you or you move chair, or you get your coat and you go.
In my world I would probably not expect my dh to do the sticking up for me because he is quite quiet! But it is worth giving him a chance to step up if that is what you want.
If he cannot, you do it for yourself but do warn him, "Your failure to help me in basic every day situations make me wonder what you would be like in a crisis!" Try and have a neutral chat first. Hoe are things going, how his mum is making you feel. Avoid calling her a bitch, it will just enflame the situation.
As far as the all day drinking goes, is this once a year or once a month, or once a week. The one a year doesn't sound too bad, (not my cup of tea but OK for some) but if it is more frequent just make it clear you won't be joining them.
And ignore any comments here that are upsetting. AIBU gets brutal. But this is your life, take charge, find your ovaries and be strong!