Dad refusing to come to wedding without his new girlfriend.

Sending you a huge cyber-hug - what a horrible thing to have to deal with.

I am so, so sorry you are going through this. I think what you need to hold on to is that this is NOT your problem. Your father is the one who has chosen to throw a spiteful tantrum to try to get his own way. Please don't give in to him. I know it's really hard, but to be blunt, if he is selfish enough to present you with this kind of ultimatum, then he's not really the best person to be giving you emotional support on such an important day anyway. It is totally reasonable and understandable not to want a very new girlfriend that you don't even know at your wedding. Quite apart from how painful it would be for his ex.

As for who will walk you down the aisle - who else are you close to? It doesn't have to be your father - it can be any close relative or friend - whoever you feel will be a really supportive and calming presence for you. Or, if you feel that asking anyone else will just highlight your father's poor behaviour, then you could walk down with your fiance. We did this when we got married (actually because Covid regs meant no one else could be within 2 metres of me at the time, but I was actually really glad we did it that way. It felt so right) and it worked out really well. You could always meet your OH at the entrance if you want to have that 'first look' moment.

How awful to put you in this position, and not the way a father should behave. This is your day, and you are within your rights to invite who you want. Your father is being incredibly selfish, this lady has never met you or your family so why on earth would you want her at your day, and potentially causing a scene with your father's ex, who as you say, has been a huge part of your life. As RGS said, this is not your problem, he has put you in this position and you must not give in as you will then spend the whole-time from now up to the wedding stressing. Try and speak to him to explain it is nothing against her, but he needs to respect it is your day and you don't want any negativity or people who you don't know at your wedding. Personally, I wouldn't, but if you feel you need to offer a compromise, can she come to the evening? My worry here though would be how it would make the ex-feel, and you consider her feelings above this other woman.

How long until your wedding? Would you maybe reconsider nearer the time, as this is a new situation so you may have met her by then and the dust settles a bit more on the split, or it may be irrelevant as they could have split by then. That said, if you are sure, you don't want her there, forget that and you do you, is your day. Is such a hard decision, but you must not back down, stick to your guns. You may have to accept that your Dad won't come, which is heartbreaking to realise he is putting her before you, but as hard as it may be, your feelings on your day come first and if he can't respect that then you are better off without him being there, but i suspect he may change is mind nearer the time if you stick to your stance and say no. I wish you all the best, keep your chin up and have a large glass of wine XX