I made a mistake that destroyed my relationship

I ruined my fiancé and I’s relationship and I need help coping and finding forgiveness for myself in the process. I lost the love of my life because I made a stupid mistake. A month ago I went to my local gas station where I know someone who works there, a young man my age. We sometimes chat when I go there. One day I was getting gas and I stupidly told him I was going to an international market (from the country he is from). He became enthusiastic and asked if he could hitch a ride. Immediately I knew deep down it wasn’t a good idea, because my fiance would not feel comfortable with me having a male in my car. I told him no and said the seat was too messy. Hoping he would back off, I got in my car, but he persisted. He said it would be really appreciated if I could help him get groceries since he does a vehicle. I caved and said okay, because I truly want to help people when I can. The ride was very uncomfortable for me and even thought about telling him to get out of my car, because my partner would honestly be so angry and knowing this, it made me so nervous. We stopped at a thrift store and I donated some things and then we each got separate groceries at the market and I gave him a ride back, dropping him off in a busy city intersection. He was very polite and respectful, and I have no feeling toward him because I do not know him well. honestly I do not know why I made such a dumb choice whether out of wanting someone to talk to (I have very few friends) or what. I did not tell my boyfriend this information immediately but I don’t think it’s healthy to hide things or feel like you have to hide things from your partner so I told him. It cost me my relationship, because my fiancé does not believe me that “something” didn’t happen and is livid that i let someone into my car. I realize it made him uncomfortable, and was a mistake on my part, but I had no intention of being unfaithful to my fiancé. Yet it cost me everything. Our marriage is off, and our relationship has a wound of mistrust. I feel I shouldn’t be so punished so much. Trying to make sense of it.