What do I do if I see my ex tomorrow?

Tomorrow, I fear it is very likely I will run into my ex at the avengers premier tomorrow. We broke up back in September officially, but it was an exceptionally painful and difficult break up. I feel I have moved on, and I dated a girl for about 5 months, so I feel good. But I'm not sure how I will feel or think if I see my ex tomorrow.
We dated for almost 5 years, and went through a lot together because we were together when we were 16. I know when she called off the relationship she was very conflicted and I'm not sure where she is at this point.
Anyway, if I see her tomorrow what do I do? Like try not to look at her, make it seem like I didn't see her, ignore her if she tried to talk to me?
I'm just worried I'm going to take a massive step backwards, but I don't want to miss this premier.
What do I do if I see my ex tomorrow?
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