I slept with my ex on tuesday
Why did you decide to do that?
I was a little bit drunk
It's not my business to say so, but you may regret that decision in the future as it is your ex and there's a reason why they're your ex. I suggest that you talk about the incident with them and make sure to make it clear whether it's just a one off or if either you want to have a relationship again.
It's totally fine. I want to move on but I just lack self-confidence.
In that case, I can see that this was just a one-time thing, and probably didn't mean anything to your ex.  I'm sure that your other friends can help you to find the confidence to meet new people and form new relationships, so you should probably just avoid seeing your ex in potentially-intimate settings like this.