Feeling guilty of something I did

i am sorry you are feeling like this , let me help you..

the first thing you need to understand is why you are feeling guilty, and the answer to that is , thoughts, when you are entretanined in your favorite hobbies do you feel guilty about what happened? certainly not , since you are not thinking about that, so first akknowledge that the situation it self is not making you suffer but just thoughts , what king of thoughts? well , you think what happened was bad for him, and that it was your fault..

lets examine this thoughts, are they true?

do you need its bad for him in the long run? maybe thanks to this he will be more carful and get a promotion or something, its definitly possible , so you cant assure its bad for him, and the part of blame, well you acted based on your thoughts, but do you control your thoughts? what is the next thought that will popup ? will it be something about the pass, the future? we dont know, if we controlled our thoguhts we would always pick positives ones, but that is not the case, so in the moment, you felt that , that was the right thing, so stop blaming your self, you had no control over the thoughts that popped up in your head...

I think you might be right: "I ignored them and wrote my complaint in anger" does sound like it might be some unfortunate behaviour.

One of the benefits of 'virtuous' behaviour is said to be 'a lack of remorse'. So if you're felling remorse now, then maybe that behaviour wasn't virtuous.

There are some hints here about how to define 'right speech'; for example,

"Monks, a statement endowed with five factors is well-spoken, not ill-spoken. It is blameless & unfaulted by knowledgeable people. Which five?

"It is spoken at the right time. It is spoken in truth. It is spoken affectionately. It is spoken beneficially. It is spoken with a mind of good-will."

Another sutta says that what you say ought to be true, factual, beneficial ... and that, although it might be agreeable or disagreeable, in either case, you need to "have a sense of the proper time for saying them ... because the Tathagata has sympathy for living beings."

I think if you're angry it's easy to justify angry speech: "what happened is wrong, it's right to tell them, I need to tell them that so that it doesn't happen again."

Anger however is one of the things which can cloud/distort your judgement (of right and wrong).

So if or while you're feeling angry might be one of the better times to not say too much, instead think about it again later if at all. Maybe anger used to be useful, in the animal world for example: if there's some struggle for life and death, anger might make you strong and quick (maybe it helps you to over-react physically). But seeing a mosquito in the ice cream doesn't sound like that kind of emergency.

The feeling you are getting is just a feeling. This is aggravated by continuous thinking as to I should have done this or not done that. (E.g. not writing the complaint, just only making a verbal complaint, etc.) This additional thinking is rubbing salt in your existing wound of remorse. Best way is to cut it out by focusing on the breath or another chosen object. (Nīvarana,pahana Vagga)

Also another way would be is just to concentrate on the bad feeling keeping in mind that this also will pass away, i.e., look at the arising and passing or the impermanent nature of the feeling. (Pahāna Sutta, Avijja Pahana Sutta 2)

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 & Dhammanupassana: thinking... thinking... regretting... regretting...