Second-guessing a relationship?

Has anyone ever been in a relationship that they know is wrong for them, but still love their SO? Or had second thoughts about the relationship for whatever reason? If so please explain.

Maybe I should explain my situation a little more. I have been engaged to my best friend since I was 19 (now 21) but he has a child from a previous relationship. He is also very controlling toward me & I almost feel trapped at this point. I crave the single life for the simplest reasons; being able to hang out with my friends whenever I want, for example. But I do love him & the child so much. I fear losing them, but I also fear the regret I may feel in the future for choosing to hold myself back because of a relationship. Any advice is appreciated!