How to create a basis of trust as the basis for a relationship again?

I (f / 17) have been with my current friend (m / 24) for over a year. Yes I know the age difference is big and yes I know many of you will not approve of it but that's not the point. I know I'm still very young to say that, but I love this man very much and have always seen him as the man I'm going to marry. So far, our relationship has worked very well. However, also have a long-distance relationship (2 hours apart) if this plays a role in the following.

But now to the real problem:

We recently spent our anniversary in a wellness hotel (as a mutual gift) and afterwards he was at my home for a few more days. We were in bed watching Netflix when his cellphone came on because he got a message: "Do you want to send me a video?" I saw this now and told him he got a message but I would like to see it with him. At first I thought this was some bug from insta but when I unlocked the phone there was a whole chat history. He started with this girl sending him a picture and he only replied "look hot". Not much more happened in the chat but he wrote to her again and again and asked her if she would like to write with him (perverse logical). However, the two never capitalized. But when she asked what the planned meeting would be like, he only replied "I would be looking forward to it" and on our anniversary he wrote to her "Can you get in touch if you like". My biggest problem is that I was lying next to him in bed for much of the time that I was writing, especially on our anniversary. When I read this I was horrified and didn't know what to do. We're still more or less together at the moment but I have to constantly think about this situation in which I lie next to him while he is writing to her. I just can't get this picture out of my head and for me it's an insane breach of trust. He assured me quite credibly that this meeting would never have happened and that he only wrote to her to keep the conversation going. I don't know what to do. On the one hand, I really love him so much, otherwise it would be over long ago, but on the other hand, my trust is completely destroyed. Do you have any tips or have any of you ever been in such a situation?

You don't seem to be the only one. This is always the problem with long-distance relationships if you rarely see each other. There's a lot of free time.