Feeling guilty of something I did
Getting angry is certainly wrong. If this is the first time it has happened to you in that restaurant, a verbal complaint would've been enough. Waiter apologizing and replacing the ice-cream is a clear indication that he took it seriously. Written complaints are usually used for intentional errors or when they don't take you seriously after complaining.

To deal with your sadness, do Vedananupassana: Note the sadness as sad... sad... sad...

To deal with regret, do Cittanupassana &amp; Dhammanupassana: thinking... thinking...  regretting... regretting...