SoonKeong:
What I'm gonna say next is not going to be easy to do, and will even draw disapprovement from many here. But its what I think I would do if I'm in your position. (Actually, I have a similar experience of my mum being unreasonable).
1) Talk to your mum nicely what you feel.
2) Most likely, halfway thru, you'll be quarrelling, or feel that she is being impossible.
3) Then just blow it out. Quarrel like mad. Don't need to with-hold too much, but no vulgarities, just say out whatever's on your mind (but of course not things that will hurt her badly, or insults, like maybe say she stupid. but more like, " You always want me to do what you want, but you never thought of what I wanted", that kind)
4) Then, cool down. Go out.
5) Go somewhere else to stay for the night ( or several nights).
6) Call home before 12am, to tell her you are not going home tonight or for few nights.
7) Give her all sort of excuses why you don't wanna go home.

After a few days, go home and see if she has sort out her thoughts.
I know some will think that its bad advise, but, seriously, how would she know the importance of listening to her son, till its too late? I'm not saying that mothers should let their son do whatever they like, but, parents need to know that children do grow up to become adults, and when their kids become (or is at the stage of becoming one) they no longer control them. They become their children's advisers. They can advise, but, they can'y control. If they do not learn their changing role, its inevitable that their kids will sometimes hate them for not understanding (who can tell me you have never hated your parents before? not very long, but for that split second, you feel that you didn't love them?)
its not easy to let go of your cutie little pie, but, parents will have to learn to let them go a little, so that they will come back. everyone needs space. their kids need space, they need the space too.
going away for awhile will make her sort her thoughts out. my aim for the above actions is to let parents realised that they cannot tie their child to them forever. The more they are afraid of losing their son, the faster they gonna lose him. if they give him a little more understanding, their son will definitely come back to his mum and remember her as a understanding mother.
true, maybe your gf and you may not be forever. but, i'm sure she will not be the only one in your life in the case where the both of you can't make it (I'm just saying if, not cursing you) the next one will come, and your mum still need to deal with it anyway. she will still act the same way, until something drastic happens. rather than watch you start to hate your mother day-by-day, and at the end of the day run off without her, why don't you do something to let her know, she is making life unbearable for you, and you really don't wish to go, just that she needs to give in a little, and you give in a little?
*Throw rotten eggs at me, I don't care. I won't take this down. I rather being labelled unfilial, than
acting unfilial. I've been there done that, I don't hate my mum, and she understands my stand, and i understand hers as well. *