How do you cope with having to spend time with people you don't like?

Hi, hope everyone is doing good.

I'm in a tough situation at the moment. My Mum has this boyfriend who spends a lot of time at our house, and I am forced to spend time with him and do things with him. My mum is forcing me to be around him and 'build a relationship' with him. But I do not like this man. He has emotionally abused my mum, and apparently he has apologised and now changed and not done it for a while, but I can't seem to get over it and forgive what he did to her. This is the biggest reason why I don't like him but I also just don't like how he's so full of himself and always thinks he knows everything, and he has a really annoying sense of humour and is sarcastic ALL the time. How can I make it easier to spend time and be around him, which I have to do? I feel really trapped and I feel like I have no options. Spending time with him is really difficult because I feel like I have to be really careful of what I say in case he takes something the wrong way. I can't be myself and I am constantly having to smile and pretend everything is okay so that he doesn't take it personally and get upset/mad at me or my mum if I don't look constantly happy. I think it's linked to my strong sense of justice in that it feels really unfair that I'm expected to just forgive him for everything and I feel like I shouldn't have build a relationship with him becauase he doesn't deserve it. But my mum insists that he has changed and he isn't abusing her any more. This may be true but I still can't seem to get over it because in my head, if he was a good person or truly cared about her, he wouldn't have done it in the first place. Any advice? Thank you in advance!