I feel sad not happy and it's my girlfriend's fault
Why do you think your girlfriend is responsible for how you feel?
She gives me plenty of reasons for feeling sad and not happy.
Maybe you should not be with your girlfriend if she makes you sad and not happy. It is not very good for your mental and physical health to be with someone who brings you down.
I just don't feel very happy around her
If you feel unhappy in your relationship, you should activately work to understand why. It will benefit the both of you to communicate about what you are feeling.