It can help to write out what you really want to say in advance. Then read it a few times until you're sure of what you want to say.

Start by letting him know that this is difficult for you, that you're afraid, and that you need him to listen with out interrupting. Explain that you really aren't in a good place right now, and that you want to be single. You need space, and his affections are starting to bother you. His actions make you uncomfortable, and you need them to stop.

As for the greater problem of not expressing yourself, you definitely need to work on that in therapy, but for now maybe you could start by keeping a journal, or a blog. It's easier to talk to people online because you have the protection of anonymity. It can be good practice for speaking out in day to day life.