How do you convince yourself to do something, that you want to say
Decide. Don't decide and re-decide. This takes a bit of will power, if you find yourself ruminating stop and think of something else. It can be helpful to write down your decision so you can read it if feeling doubtful.
tell your mind and keep it there keep saying you can do it in your head and you can do it or read your favorite motto and keep it in your head
You can try to think about the positive consequence that your statement will make to the recipient. You can also say it in order to get it out of your system and avoid any guilty conscience feeling.