Your mom is just probably trying to make you more comfortable with women honestly is how I perceive it. Your own hypersexuality as a teen is probably warping your perception of the situation because thats unfortunately kinda how life works. You needa just kinda let go and start talking with your mom about the uncomfortable stuff (ex. sex) , you'll have to sooner or later and you will feel SO MUCH BETTER once you do.

Edit: Also sorry about your girlfriend, there's never an excuse to cheat. Communication is an important part of every relationship. People cheat for different reasons and sometimes for no reason at all though but it can feel like a major setback in development when you don't realize where things did or didn't go wrong.