After dating my girlfriend for 3 years she still won't accept my marriage proposal, I am so angry with her 
Ouch that's rough. Maybe she just isn't ready. 
I think she is secretly cheating on me :(
Do you have any proof? If so you should bring it up to her.
She has been quite demanding lately, she wants to eat out in the most expensive resturants, she wants the most expensive cloths and jewelleris 