I'm going to cancel plans because of cold sores. They hurt so bad, have you ever had one?
I never have, but why are they making you want to cancel plans?  Is the pain too bad for you to go out?
I feel it's an easy way to tell women I'm not interested in them, although it's a real thing that happened to me too.
Even though you don't like those women, it's not right to cancel plans which you already made for them. They'll be disappointed.
Here's the thing: I can do whatever I want. I feel like other people are entitled to nothing. If they don't like it, they should stop breathing.
This is a needlessly harsh attitude to take toward others.  Just try to put yourself in the other person's shoes, and think about how it would feel if someone suddenly canceled a get together that you were looking forward to.  If you are not feeling well, or are not interested in going out, then at least be honest about it.