Trying to build friendship with ex

So my ex and I broke up 3 months ago and since my school offers retreats I decided to sign up for one, they are relatively small retreats (15 people). It turns out my ex is going on the retreat too because she singed up towards the last moment. During retreats in small group people talk about what's been bothering them and for me its social anxiety and trust issues, stemming from when we broke up she told me that when I would ask her if she wanted to hang out she would say she was busy but in reality she just really didn't want to. It hurt me a lot though I still want to have a friendship with her because I still have a little over a year and a half of school, I feel like it is the right thing to do to try to be friends and treat her like I would any one else, and all and all just because she broke up with me doesn't make her a bad person, she still is a good person. I haven't talked to her besides for the occasional quick "hi" in two months. I just don't know the best way to approach this. I think the best thing to do is to sit down with her one on one see how she is doing and tell her how I feel though I also don't know the best way to word it.