Why did I wind up in dating abuse?

I am from a country where dating is taboo. I was not in a relationship till my early twenties. The following may be hard to understand but I need to talk about it and know why I let this happen to me. It was my first relationship and I had little idea what to expect. Things were fine till my bf learned another man was interested in me and I might be interested in him too....

I'm trapped in an unhealthy relationship and don't know what to do.

I am 17 now, and started dating this one fellow when I was fifteen. At the time he was 44. Of course, now he's 46, but that's not really the point. He's divorced and has two kids, one son 2 years younger than me, and a daughter the age of my own younger sister (12). I look after them for him sometimes. I feel like I really love him, but I don't really feel the same way about him....