<human>: 26M, 20F.  1.5 Year relationship.

My girlfriend has been good to me.  However she is very insecure (exBF cheated on her) and is constantly questioning me about my female friends, even stalking them on Facebook.

I have definitely been the one less involved in this relationship.
  
But recently i have this sinking feeling in my stomach that she uses guilt / head games to manipulate me.  I am so insecure now that she's cheating on me.  I'm becoming the psycho stalker boyfriend.

I talk to her about it and she denies any wrongdoing.

From the outside and her actions, she is the perfect girlfriend.  But for some reason my gut is telling me to let it go.

I myself have some trust issues, is there any way to know for certain?  Any red flags i should be on the lookout for?

Can you summarize the given text in a few sentences?
<bot>: 26M, 20F, 1.5Y.  She was cheated on.  She's insecure.  I'm insecure.  Is she manipulating me?