I was engaged to someone for a year-and-a-half, after which he decided it wasn’t working and ended it. I took it quite badly and was heartbroken when I found out that he had married someone a few months after we broke up. I know it’s normal for relationships to end, but I have been struggling with the way he moved on. I don’t know how someone can be engaged, promise to be with someone for life, and then find someone else so quickly. It’s making me wary of even going out on dates again because it feels as if I can’t trust people. How do I cope with this?
You have to try and sit with what you’re feeling and accept that people can and will cause pain whether they want to or not. Your grief is real as is the inability to trust someone you once loved, but you can make things easier for yourself by acknowledging that this is a process. There is no shortcut when it comes to managing grief, and all you can do is be kind to yourself. Take as much time as you need, accept that you’re probably not in a position to start dating again just yet, and try spending time with people you like being with. Trusting someone again will take time, so the best thing you can do is take small steps until you feel strong enough to take that kind of risk again.

Am I being naïve by trying to be with someone who has a girlfriend? They are in a long-distance relationship which is the only reason why I spent time with him, in the hope that he changes his mind about her and chooses me.
You are wilfully trying to get someone to end a relationship in the hope that you come across as a more viable option. Think about the fact that there are other people involved here, and whether it is morally defensible to hope for the worst in their relationship.