My (m33) GF(f25) slept with someone on a break

My GF (25) slept with someone whilst we were on a break. Relationship was very up and down and agreed to have some space. Space lasted for about 14 days and then we reconciled.

We were sat on the sofa chilling out when she received a DM off a man saying “where have you gone? Just hump and dump”. We got into it and she lied for a while before admitting she slept with this man whilst we were on a break.

This man is someone we have argued about throughout the relationship and a previous FWB of hers. He is much more well endowed than me (she had a picture on her phone at the start of our relationship) and always bothered me as it made me insecure but she reassured me I was the best Bla Bla Bla.

I can’t get over that she apparently was in love with me but be intimate with someone else within a two week period and more so how she rushed back to someone bigger than me. She claims it was a mistake and wasn’t thinking straight but I just can’t get over it. It’s been a month and its still at the forefront of my mind.

Am I over reacting? Should this be something shit that I just get over?