He's not over his ex. Should I be patient or should I go?

I met a really amazing man at a mutual friend’s bbq and we hit it off immediately! We talked for 6 hours and started dating that week. We truly had an awesome connection and started spending most of our free time together. Things seemed to be progressing beautifully until 6 weeks in. This is when, on our first weekend away together, he breaks down and tells me he lied to me when he said he was over his ex. I had asked him about it because he casually brought her up a few times. So now I’m feeling heartbroken. I know he cares about me a lot, but I don’t know if I can be with someone who is still missing his ex. They dated for 4 months and were broken up for 2 when we met. He says he’s not over the way it ended between them (she broke it off without much explanation and moved away), but I don’t think he’s over her altogether. I care for him deeply, but I don’t ever want to be or feel like someone’s “second best”. I don’t know what to do. Please help!