"I feel kind of ashamed because I don't really have any control over it and I dont want it happening, I feel like I'm betraying her trust"
You see the inherent contradiction in this statement, right? You're not actually betraying her trust, because dreams are not something you are doing, but something that is happening to you. It's not something you have control over. Promising not to have dreams involving other women is kind of like promising you'll have a bigger dick, or have naturally different colored eyes, or whatever.
Also I think it's completely normal to have sex dreams. You are in a relationship. Presumably you are having sex. You also see other people throughout your day. Dreams are just amalgamations of what is on your mind and your past experiences.
Don't get caught up in the "thoughts are cheating" thing, it will just put unnecessary stress and anxiety in the relationship. Being in a relationship does not mean you must be in denial of other people's attractiveness.

Do you love your girlfriend? Do you find her sexually attractive? Do you want to be with her over other women?
If the answer is yes to all of these, I don't think you have anything to worry about.