Aio? Insecure in relationship

Story:-

When I was in school, I had a friend named M, and his girlfriend was T. They were in a good relationship, but after some time, a girl named F, who was in the same class as M and called him "brother," shared something. One day, F told M that she had seen T several times on different streets, at nice places, and restaurants with another guy. When M asked T, she said they were "just friends," but F mentioned she had seen them together even at night and many other times, eating and hanging out. It didn’t seem like they were just friends.

When M found out the whole story, he broke up with T.

My question:-

I don’t understand why some people cheat. If you like someone and say you love them, why would you cheat? If you’re not feeling the same way anymore, why not just have an honest conversation and say, "I don’t feel the same, so I think we should end this"? It’s that simple. Why say "I love you" and then betray someone behind their back?

Because of situations like this, I sometimes feel insecure in my own relationships.

For example, if I’m busy and can’t give my girlfriend all my time, and she’s free, I start overthinking. If she talks to another guy, I feel insecure. I know she probably doesn’t have bad intentions, but as a guy, I also understand how most men think—they often want to form a relationship.

If I bring this up to her, I worry she’ll think I’m doubting her character, but that’s not what I mean.

So, I tell myself one thing: If she’s truly mine, she will always be mine. If she’s not, then she wasn’t really mine in the first place.

Share your suggestions and thoughts.