Should I skip these talks because of former prof crush?

Ok, long story short: at my former uni, there was a professor (Jack) I grew to like (Ie crush on); before this,, I'd had the impression he was attracted to me, and I liked this. Nothing untoward ever happened - he just used to talk to me a lot, tease me sometimes, on faculty events (mainly in pubs/bars after presentations), on a few occasions he sat really close to me and none of the other profs would do that....

One evening, he told me that our relationship could only be 'an employer/employee' relationship because he was a prof. I understood this: he had his job to think of (a colleague of his had joked to me that Jack "knew everything I was up to!"...). Since then, Jack sometimes ignored me, but sometimes was friendly.

The final two weeks of term, we ignored each other totally: he looked me straight in the eyes one day, but I just looked away and down at my phone, because I assumed he'd want to avoid me. We never spoke. I once heard him say hello to me but I rushed past too quickly to reply. I emailed him to ask his advise about choosing Masters courses, but he didn't reply (I know that he saw it).

Anyway, now I am doing an MA at a different institution. Now, Jack will be teaching a few supplementary classes at my current uni, and I have to (well, I am strongly advised to) attend. The problem is, on one hand, I really want to see him... On the other hand, I don't want to see him at all, because I feel hurt and angry and it might be painful to see him...

No reason not to attend a recommended course or feel hurt and angry.

He handled things very kindly and professionally and most of all, was ethical and did not take advantage of a crush.

he told me that our relationship could only be 'an employer/employee' relationship because he was a prof. Jack will be teaching a few supplementary classes at my current uni, and I have to (well, I am strongly advised to) attend.

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If is strongly advised that you go, and if you think you can get something positive out of it, I think it would it might be best for you to just suck it up and go.

He's still teaching, the roles are the same and the result will be the same (nice to hear about someone with ethics). However, you both are adults and you say his courses are highly recommended for your MA program. I would assume that means the course content will help your learning and that translates to your future career. What is best isn't always pleasant, so put the crush stuff on hold and focus on what you need for your education.

Don't pout your way out of an education.