Advice for my letter?

I’m thinking about writing the letter, but I’m not sure how to go about it really… i’ve already written out an apology letter, it’s kind of long, just over a page. I admit my mistakes and tell her I’m sorry…

Do you guys think I should make it shorter though? Short and to the point?

Also I wanna put something like this in. “You once told me that not being with me would be the biggest mistake you ever made. The biggest mistake I will ever make is pushing you out of my life by being so stupid.”

Do you think that’s a good idea, or should I leave that out?