It's going to be a bumpy ride

[Dom R]

It has been said, that "wayward" spouses, have very short attention spans. So, whatever you decide to say.. keep it short short short. No long elegant logical arguments.
And ESPECIALLY no "I'm changing" arguments. everything I read says that you should always let THEM notice changes in you, not argue "i've changed".

[an exception to this, might be if they accuse you, "you always xxxx", to which you could say, "yes, i used to do that. But I havent done that for months now, have i?"]

You might even keep it as simple as,

"I care about you. I'd like to date you, and have fun together. But I will not be a doormat. If you want to date me, then decide that you want to date *me*, and no-one else during that time. It's your choice; you're free to change your mind. But if you choose to date me, i will not be disrespected by you dating other people at the same time".

(you might even expect him to turn you down.
This is a test. Accept his decision.
See if he changes his mind in a few days, when he sees you mean it )