My girlfriend, who is 18, mentioned to me that she is interested in having a threesome.

I've been through some serious issues with infidelity in the past, and she’s aware of that. I usually don't have the desire for sexual activities, which makes me worry that if I can't meet those needs consistently, she might seek that elsewhere. She has reassured me many times, and I’m starting to trust her. However, when she randomly brought up the idea of a threesome, I felt uneasy. The last time someone mentioned that to me, they ended up cheating because they didn’t feel fulfilled. It’s triggering old memories, and even though she’s said she doesn’t want that anymore because I’m uncomfortable, it reminds me of what my ex did—she promised the same and still went ahead. I’m at a loss about what to do. Right now, she's texting me to apologize, but I’m unsure how to respond. How should I handle this situation?