OK. You already know that if you keep pushing him away then eventually he will be pushed. So what you need to do is find things you can say to yourself at these times to try and manage that urge.
Simply, you can just keep your mouth shut and say nothing. That's the easiest place to start. Just don't say it. Or go into a different room/turn off your phone and say it to yourself. I've used the notes on my phone to type out the texts I wanted to send and, when I've looked back at them later, have seen how bonkers they really sound. That helped!!
Things you can tell yourself...
He is with me because he wants to be. No one is forcing him to be with me. He can leave at any time.
Because all of that is true.
Worrying about him cheating won't stop it from happening. If he does, that's on him and says more about him than it does about me
I was absolutely fine before him and I'll be fine after him
My light bulb moment came when I dated someone who was very clingy, anxious and worried about these things. It did eventually push me away. It was hurtful that he didn't trust me and exhausting because there's only so many times you can listen to the same things and offer the same reassurances. It's what finished us in the end because he couldn't live with his feelings of anxiety and I couldn't live with the impact that had on me - it made me anxious too.