Your mum is probably paranoid -
overprotective of her daughter as well as the dignity of the
family's name, fearing that should one day things just happened and you got
pregnant due to lack of proper protection or common sense.
Your mum is a classic believer of
'I-trust-your-man-but-I-don't-trust-man-in-general'.
The panacea to this plight is of course a marriage.
But meanwhile, what you could do is to expand the boundary of what
you can do and cannot do through proper dialogue, communication and
trust. She probably needs to know much more about your boyfriend
before she could decide if she feel safe putting her daughter in
his hand. Probably tons of family dinner, causal meet up in your
house, goodwill and stuff to familiarize your mother with this man
you are dating.
From there, say if you must be back before 12 midnight, work to
stretch until 1 a.m. Then maybe about 3 a.m so on and so forth.
Always let your mum know that your boyfriend will ensure the
responsibility and safety to bring you back home well and
intact.
When you rebel outrageously - chances are, it wouldn't go too well
with your mum and she might even think that your rebellious act is
instigated by your boyfriend. In the long run, it hurts goodwill
and bonds.
You want your freedom, but your mum wants security. Can you work to
balance the equation? 
Cheers
