When we are young adults we don't often realise that when we go to someone and spill our lives to them, we can get responses that isn't what we want. Simply put, if you don't want a negative reaction then cease mentioning your partner at all. Easier said than done though.
I think you hit the nail on the head when you said she is the only company you have. You need to spread your wings and get some friend your own age. Then drift slowly from your mum to a point whereby you will both be happy to see each other. If your mother realizes you are drifting and questions you "well mum, we cant live in each others pockets".
This is all part of maturing and isn't something to be ashamed of.
So I'll leave it to you to find groups whereby you can meet other mums for some developing friendships. In terms of control, I don't think your mum is trying to control you, I think she is just trying to help. I have a mum that ties to control my life totally and 9 years ago cut her free from my life. She is no comparison to what you have in your mum.
I think White Knight has been very helpful. The problem is your mother is your only friend. It's natural you'd want to vent to her about little things your partner does that are annoying for example, because she's the only sounding board you've got. But she's in no way impartial. She's bound to be upset if she thinks you're being treated not as well as she'd like, so she is going to turn against any boyfriend you have if you tell her all the nitty gritty. I'm not suggesting you give her a false impression, if you are truly unhappy or being treated badly say so, but it seems like maybe it's not that. Maybe you just need to separate your romantic life from your mother daughter life, if that makes sense.
Mother's groups are great if you can find one in your area. Or if you have a particular interest you could join a group in that interest area. It's scary to meet new people but ultimately rewarding if you can find a friend to talk to.
Best of luck