This is spot on.
The professor was outright strange from the start. Finding all the ‘spare tickets’ and taking a family man away on so many weird outings then the ‘feeling threatened’ comment.
The partner was sorting a mortgage out behind OPs back - after two weeks of not speaking and having an affair with a vulnerable woman. Whether the affair started before or after the France trip with the professor she will probably never know. It sounds like he was lining up someone from the group be it the OW or the professor. Who knows. He shouldn’t be in that group full stop so glad he’s been turfed out.
Your step dad is a legend and I’m glad you have support from your parents too.
This is not the behaviour of a ten year old relationship with someone who has respect for you. He has behaved abhorrently, using his MH to deflect from his behaviour likely during the whole ten years and preying on vulnerable women in addition to the outings with the professor abroad.
You will have wobbles, as PP say it is how the end of a relationship plays out as we grieve the loss. It’s the fear of the unknown and stepping out into that - it’s scary, of course it is. You made life plans and he was part of your family.
This man deserves no time from you OP. He has chosen this path. You will never trust him again, and as the old saying goes - he has shown you who he is.
Stay strong, know that you have support in real life and on here.
Sending you the strength of a warrior to get through the dark days, but know that you will and you will see a bright future given time.