Try to give it a bit of time, as
>>15 said you've been her son for a fair while, that won't just go away at once.
You also need to remember that you're asking her to make a change, hopefully she can be accepting of what you want but you need to make sure you listen to her as well. Strive to reach an agreement but at least come to an understanding, otherwise it will make life very difficult for everyone involved.
It can be quite a confrontational thing for people in this sort of situation, make sure you're patient with her. Don't let her put you down though, make sure things stay civil and that you don't experience pain simply because you want to include your family in this part of your life.
Ah I didn't really mean to single your mum out for this, you ought to apply it to everyone you want to involve, I think.
I hope that helps a bit ^-^ good luck with everything!~