you have already cheated. It's not just the deed but the thought.

You need to make a decision. If it's to stay with the current BF you need to cut yourself off completely from the old. And then make reparations on the second.

I'm not sure what to do about those occasions you show up to the same places. The first start would be to admit to the current boyfriend you are uncomfortable in such situations. Make any interraction as short as possible. Make it clear through your actions and even body language to the ex that you have moved on. Ensure he understands that talking beyond friendly small talk is unwanted. It's going to be really difficult the first time you do this. The old boyfriend will feel snubbed but you got to do what you got to do.