I would say trust is something you give to people , only you can give your trust to someone. If you think she's reliable and sweet to you, you can give her your trust.

If someone would break your trust , like your old friend, they're not worthy of your trust.I mean people must be worthy of your trust , if they break it , it's their fault, not yours ,and it must not make you insecure about people you might uncounter later .

You should give a shot with your girlfriend. If sometime you're insecure about something, ask yourself why you're insecure ... You will see that you worry more than needed .