why is my boyfriend so mean to me

You boyfriend is mean to you because your accepting his behaviour. Stand your ground, know who you are.

Being mean to you is a controlling behaviour, so unless you take control he will always be mean to you. 

The best way to overcome this problem is to:

Be financially independent 

Eat/drink healthily 

Stay fit and feel fit 

Always up your standards 

I understand this is a daunting time for you but it will continue if you don’t do this.