Honestly, it doesn't sound like you're ready to hang out with your ex. And that's ok. You don't have to be. If there is a fulfilling friendship to be had here, it can wait until you've healed and moved on from the relationship.

If you can't be friends with your ex if they've hooked up with somebody else, then you really just can't be friends with your ex, period. That isn't a reasonable or fair expectation to have of her.

Also- it really doesn't sound like you want a friendship with her right now, or maybe ever. You don't need to force this out of obligation. It isn't unkind of you, and it doesn't mean that can't wish her well. You can cancel your hangout, and explain that you're not ready to be friends with her at this time.