I'm uncertain about how to handle my feelings and I can't determine which choice is more immature.

I've been in a relationship with my 18-year-old girlfriend for six months now. A little while into our relationship, I discovered that she still has feelings for her ex, who is in classes with her at school. Two months ago, I asked her directly if she still liked him, and she replied no, saying she just misses what he could offer her compared to what I can right now. Then, about a month ago, her ex came to school looking sharp with a new haircut, and she admitted that she was attracted to him. It's important to note that he's been a friend of hers for most of her life and is a twin. I'm torn between letting my insecurities and trust issues influence me or having a conversation with her about how I feel. The thought of her still having feelings for him while they talk nearly every day really bothers me, especially since he was somewhat toxic in their past. I know my age might be impacting how I view this situation, but it still hurts to hear that she likes another guy. Should I confront her about it, or just keep trying to navigate through these feelings?