How can I stand up for myself at an autistic social group with my ex there?

I’m attending an Autistic social group in a few nights’ time. There will be lots of people who matter to me attending. One who is a man who tried to use me for attention and freebies, with whom I cut contact last month, might be going. This is a new situation for me and I’m not used to standing up for myself.

The last time we spoke, he admitted to dating and sleeping around behind my back and using me as a placeholder. I am a bit upset about it still, but what I’m really concerned about is that I might be too nice and fall back into old habits, making excuses for bad behaviour which shouldn’t be excused; so as ‘not to make a scene;’ or ‘cause a fuss; or ‘be unkind.’ How do I grow a bit of a spine here?

How can I stand up for myself at an autistic social group with my ex there?
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