He told you that you deserve better—and he was right. You do deserve better.
You are so young and have so much of your life ahead of you—so many chances and possibilities. Why not make the most of all of those possibilities rather than wait around for someone who is trolling Tinder because “he’s an addict and addicted to everything.” (That’s an excuse by the way). It doesn’t really matter who he had sex with, or what she looks like, he has moved on to having sex with other people and I think what’s key here is “moved on” and it was his choice to get on Tinder.
There are so many of us on here, a lot older than you are, who have invested our lives with someone who has addiction. I would personally love to have back the years I gave someone who treated me so poorly, and I know that will never happen. Yes, it hurts how he acted and what he did to me and I lived (subjective term) through breaking trauma bonds, although I admittedly still struggle with why he did what he did to me, and I understand the hurt and the betrayal. And I agree with your ex, you deserve better.
Ultimately it’s your choice whether you move on with your life or you wait for “something” that may or may not happen. I know what I would want you to do if you were my daughter or sister; I would want you spared the hurt that I have been through.
I wish you all the best as you make your decision.