Best way to put it is "I'm dating a new girl now. You take care."When you get past that point is usually when they come around. My problem is when I like a girl as much as my ex, I usually self sabotage (consciously or sub consciously) any plates that I’m spinning when that happens.
Back in my 20’s I purposely threw a 1 year relationship down the toilet for an ex I had strong feelings for.
Now I would be upfront and say I’m dating someone else, but open to also dating you again; and may the best girl win.
Give her the sense she can't have you. This will trigger her panic/chase mode. She has to know she blew her chance and you do not care.
Telling her you will date her too will tell her you are still hung up on her and feed here ego. That is how she will see it.