This happened to me aged 17. ( I had been on a fortnight's holiday with my parents, went on a prearranged date on the Sunday, and noticed that he was acting strangely. On the way home he parked up and told me that he had met his very needy, ex girl friend by chance. They had talked and as she was distressed about a family upset, he offered a shoulder to cry on.
I started to say that was very kind of him. Then he dropped the bombshell - they were getting back together and I was dumped. Home from work for Lunch with my Mother on the Monday, I was crying into my Mince and Tatties, but ever practical Mum said that I had 2 choices 1) Chalk it up to experience and get over it, or 2) fight to get him back.
I went for the 2nd option, turning up to ever gig that his band was playing at or other places where I knew he would be. I stayed friends with the other Band members and their girlfriends and enlisted the help of my soon to be BIL's best pal - a very good looking, older, closet gay - and it worked, himself took me aside to warn me off this man, told me he was still concerned for me, etc.etc.
Within the week, I heard that he had finished with the other girl. and at a gig on the Saturday he came over, asked me to dance. When we got up together, everybody cheered. We went for a run in the countryside the next day. I got fulsome apologies, thanks for rescuing him from a terrible mistake, and, surprisingly a sort of proposal (said he wanted to spend the rest of his life with me, I said I felt the same and he then said 'well I suppose that's us engaged'
We were very happily married for 54 years until he died.