I want to get a problem off of my chest.
What problem do you want to talk about?
My long-term boyfriend has cancer, but I want to break up with him.
It is good to get problems off of your chest, as they can cause a lot of worry and stress.  If you are having doubts about whether or not to break up with your boyfriend, it's good to get advice and to talk it over with someone. However, this decision is ultimately between you and your boyfriend.
Nah, that is not worth it to me.  I want to cheat on him with someone else.
It does not sounds like you care about him, but you should at least break up in a respectable way before cheating.