I discovered that my girlfriend, who is 21, was cheating on me with my roommate, who is 22.

I'm in a tough spot and really need some advice. This morning, I discovered some explicit messages between my roommate and my girlfriend. I've been with my girlfriend for three years and my roommate for two. For the last four to five months, I've been feeling uneasy about how much time she spends with him. She frequently cancels our plans, there's been poor communication when they hang out, and she often gets physically close to him—like sitting right next to him and sharing a blanket. I've expressed my feelings about this to her and admitted that it makes me question if something more might be happening between them. This sparked a prolonged argument that's been going on for the past week or two. Her response has been that it's my fault for not being supportive. To add some context, I lost a longtime friend to suicide last February, and since then I've been working on my emotional well-being. I've been committed to addressing her concerns, making an effort to improve, and establishing a solid plan through therapy and pushing myself outside my comfort zone. However, when I raise my own issues, she says she'll work on them but doesn't follow through. When I found those explicit messages this morning, I confronted her about it. She expressed remorse but insisted that they’ve only kissed once recently. I struggle to believe her given our situation. I've told her that I need some space, and I haven’t spoken to my roommate yet. I'm unsure how to process all of this. I'm caught between the fear of losing two important relationships in my life and the pain that makes me feel like I never want to speak to them again.