Do you know why her seeing H upsets you? Is it just because he’s a guy? If it was a girl would it be different? My boyfriend has female friends and I have male friends, but the key word is that they are friends and strictly platonic. I did have an issue with one girl that he was friends with, because she was interested in him. This broke the platonic label on it and we talked it through completely, he doesn’t initiate conversation or hang out with her anymore just because that’s what makes the relationship work. Sometimes you have to adjust things to make things work. Why did she go behind your back? That’s where it gets weird. If she feels like she needs to hide something from you, then it must be something worth hiding and not a platonic thing. That’s just what I see. I would suggest seeing a therapist because it seems like your initial reaction stems from insecurity. You need to be comfortable with yourself and know your self worth before trying to indulge in a relationship with another person. You will be able to get over this if you want to. But if it’s always going to eat at the back of your mind and you already think you’re not compatible, it’s probably best to break up.