@St0rm,
The other guy's crush is irrelevant.
He's not part of the relationship between you and your girlfriend.
You and your girlfriend need to talk honestly about what you are both comfortable with.
Age and confidence usually makes this stuff easier, but you've still got to set boundaries that work for both of you.
You may need to accept that your girlfriend's comfort level/confidence is different from yours.
Right now, work on being happy she chose to be with you at this time.