Dear {Recipient},
Please accept my humble apologies for the hurtful things I said to you while we were {doing activity} last {day of the week}. I realized after the fact just how selfish and inconsiderate I had been. Please know that I didn't say those things just to spite you.
I've had some time to reflect upon my actions, and I can assure you that I've learned my lesson. I'll be sure to think before I speak from now on.
Apologies again,
{Your Name}