Hello. Here's my advice. To move beyond your feelings of jealousy you are going to have to bravely confront and overcome your own feelings of insecurity. Jealousy and the fear of being cheated on is something you are carrying around within you. Which means, nothing your girlfriend does or doesn't do will actually make these feelings go away. We often think that other people should change the way they act to make us feel better; but this just makes them feel manipulated, controlled, and eventually resentful. Plus, the truth is, she can't make this issue go away for you. You have to do that.
It is not surprising that your feelings of jealousy and fear are coming up with this woman; that is what happens--our issues emerge with the people we love.
I would start by exploring within yourself your fears around being left behind and losing love. These fears are probably something you have felt in other relationships, so I would get really clear on when and where you have felt similar feelings, and try to understand where this experience came from. This first wound of losing love has now created a fear within you that it will happen again. And because you carry this fear with you all the time, it becomes a self-fulfilling prophecy. You project your fear onto your partners and can't seem to get passed it.
YOU are going to have to soothe this part within you that is scared of losing love. You are going to have to catch yourself when you start to get jealous, and make an effort to combat this old fear-based belief. If you don't start confronting this fear, it will continue to show up again and again, in this relationship and in others too. And when you do confront it and start to move beyond it, your experience in relationships will get much, much better. A therapist can be of great help to you to move beyond this place where you are stuck. You can do it.
I made a comment on Your post in the Relationships forum so I don't need to repeat myself, but
I would like to add
that having been cheated on in the two relationships You mentioned in Your other post would certainly make ME feel gun shy this time around with a person who "appears" to be so casual about sex.
again, I speak on a personal level - but it's my feel that I would not have sex with anyone who's toothbrush I wouldn't use - not a bad rule when You think about it.
Whatever You decide to do here, I wish You well.