We’re really happy together but after this I’m not sure I can carry on with the relationship as it will be always hanging over us that I was banned from the wedding.
YANBU to be distraught at this. It must feel horrible when it’s been planned for so long.
Are you ‘banned’? Or has your DP told you he wants to minimise the huge emotional blackmail his DD is facing about her late mother, and the stress this is causing, so soon before her wedding day?
Is it fair to end the relationship because your DP was forced to choose between supporting his DD on her wedding day or you feeling excluded? Surely, his DD comes first?
IMHO it may help to change your mindset and offer not to go (your decision: you are in control) rather than see it as being banned (someone else’s decision: out of your control). It would be to make things as comfortable as possible for your DP (who you love) and his DD (who you accept must always be his priority, given he’s her only parent). If you bow out gracefully, you’ll have been kind and reasonable and not added to the stress and drama.
The bride’s maternal family have caused hurt and harm either way by their unreasonable, last minute demands. Don’t stoop to their level by making a fuss or blaming your DP.
I’d plan a lovely day for myself doing all the things I love. And then ask DP if we could arrange a little celebration for DD and her DH after the wedding: special meal etc. Your relationship will end up stronger with DD than that of the distant, demanding relatives, despite their refusal to accept you.