If you allow yourself to be paralysed by "But what if it's a mistake" then you end up never doing anything! Sometimes you just have to feel that fear but go for it anyway. If it's a mistake - that's a shame, but you can learn from it.
I don't know if your DP is controlling from what you describe, or just a bit crap at being supportive in a relationship. It's perfectly okay for you to challenge that behaviour and ask him to see your point of view. My view is that when you live together (and especially when you have a child) you should be working as a team. Not one person in charge and the other doing as they're told.
Because of your upbringing, you have been used to someone being abusive (your bro) and that being accepted within the family. So being with someone who isn't physically abusive feels like a massive step forward. However, he doesn't have to be hitting you for it to be a bad relationship.
What is your living situation? Do you rent? If so, is the tenancy in both names? I'm trying to get a sense of how things would work out for you if you did decide you were better off alone. It's a shame your mum is not to be trusted. Do you have any other relatives (aunty, nan, cousins?) who would be supportive if you were to leave?
By the way. My dad was an abuser and to this day my sister is scared of the dark and cannot sleep with all the lights off. She is 38. So don't be embarrassed about that. If you want to use a nightlight, so what?! That's your business and you don't have to explain it to anyone.