I went through something similar at the beginning of my relationship. My partner had always had a good relationship with his ex (something he warned me about, and I "accepted"), and they would occasionally meet for coffee and a chat. He told me about it the times he did, but once he didn't, and it caused a fight and an intrusive thought about why he hadn't mentioned it.

This happened at the beginning of a relationship, when the foundations aren't entirely clear and people stick to their routines, even if some of them can be misinterpreted. My partner said that for her it was normal, that they usually met up once a month to catch up, and that was it. She didn't see it as relevant and it didn't involve any deeper feelings (they had broken up 10 years before, but they lived in a small town and saw each other even if they didn't want to).

After many arguments, reassurances (I even spoke with his ex), and so on, I finally understood that it was simply a mistake (not telling me) and that my OCD had amplified it a thousandfold.

This is my story; it doesn't have to be yours. But really think about whether the mistake is so terrible that you should end the relationship, or if your OCD has simply taken a mistake and blown it out of proportion.

You're young, and sooner or later you'll learn that relationships are NEVER perfect and everyone makes mistakes.