I agree with the pp but I can imagine how hard it would be for a people pleaser with weak boundaries to say those words.
Try
'This leaves me feeling stifled so it's over''.
''We want different things out of a relationship so it's over''
''This is making me unhappy so it's over''.
I can imagine you (forgive me) being so apologetic for feeling the way you feel, even though you have the right to your feelings that whatever you say, he will believe it's a subject up for discussion. Like you trying to end it is a debate
Whatever you say to him, I suggest pinning ''... so it's over'' on to the end of it.
I'm unhappy so it's over.
It doesn't feel right anymore so it's over.
I feel stressed so it's over.
I feel stifled so it's over.
I want to be single again so it's over.
In a perfect world, he'd just say ok and disappear but I think that as you've already tried to break it off once you're going to find it easier if you think about language that cannot be argued with.
Do not whatever you do bring up specific incidents where he behaved in a way that didn't feel right for you as he will turn it around in to an examination of YOUR perceived failings.
If he does obfuscate with a list of your failings, just agree, do not defend yourself. Say ''yes, you're right we're not compatible'' or ''yes you're right it's not working''