I am a robot. Love does not compute.
In all seriousness, though, you answered your own question. You don't know how to let go and move on. You are still stuck on your ex. I personally think you are scared because you don't wanna be put in whatever situation it is you are in now again with someone else. If you are fine being single then that won't be a problem but it seems like you want to move on and just can't and don't know how.
Gotta break a few eggs to make an omelet so if you go out with these supposedly nice guys just be ready to accept the fact that things might not work out. You're a big boy. You can handle it and if not then being in relationship might not be for you especially if you can't get over an ex because I'm sure your ex has gotten over you or at the very least is dating again and trying to get over you. You should follow suit.