Struggling to make a decision

It has been four years this year since my mum died and my dad is getting married in August. I am really struggling with the fact my dad is getting married again to someone else. The wedding is in Glasgow where he lives now and he wants me to go. I’m getting emotional over this I don’t really want to go but I feel like I want to be there for my dad but the thought of him saying his vows to someone else and then the speeches afterwards it’s a lot to take in. I feel if my parents were divorced it would be easier but this is not the case. My dad says he still loves my mum but has to move but I don’t understand why marriage why can’t he live with his partner and not get married. I spoke to my dad yesterday and he says I have to decide within the next few days and it’s a huge thing for me. I’m emotional all the time I keep snapping at my friend Rikki and a week ago I had a mini autistic meltdown in front of him and then I just burst into tears luckily Rikki was able to comfort me and calm me down. I feel it would be easier if someone came with me but sadly Rikki can’t and my other friends again can’t make it either. My sister will be there so I guess that’s something I just really don’t know what to do.