Maybe explain to your mother that its important to you now, and you want your husband (IY”H) to be behind you in regard to the length of skirts and someone who wont agree to go out because your skirt length is too long is probably not the type of person you are looking for… Sorry if this was already posted.
I, (one very little person) say stick to your guns and respectfully try to explain (again) its importance to you. Hopefully, soon she will realize how much you want it and you will be able to do it with a full heart. Again, I’m one very little person. Hatzlacha!!
The other option I see is for you to get engaged and married ASAP to someone who feels the same way as you. (That was a bracha.)