many thanks to those who commented.
i think, the reasons his mum doesn't like me are as below:
1. they are from northern part of China, while i'm from the South. We have many different values.
2. his mum thinks that i'm too short for her son. His son is not tall, but his mum wants to have a tall daughter-in-law so that their future grandson/daughters can have a better height
3. His son thinks that i'm not as pretty and attractive as before, and he prefers to have a forever beauty (at least as long as she can be).
4. his son is kind of a man who likes to control everything and easy to get mad. But i tried to bear his temper, since i hate to argue with him which would just make things worse. However, in his mum's point of view, i should always bear his temper no matter what happens. But, once, i happened to seriously argued with his son in front of his parent once. Actually before i fighted with his son, i knew i should bear, especially his parents are here. but i couldnt' help myself fighting back. since it's much beyond my bear point. This made them very unsatisfied since in thier opnions, i should always bear, bear, and bear (i meant, 'Ren' in Chinese, in case i spelled it wrong).
I think again and again these days regarding our relationship. i could not even sleep at nights. And i recently keep having serious headache due to this reason. i'm really sad since we've been togehter for so many years, but now i can see that many things have changed. I thought i could let go, but in fact it's hard for me to do so.. i sometimes hate myself that much since i'm too 'Ruan Ruo'
Maybe some of you are rite, i should walk out of this relationship and restart my new life.