In this context, it usually means something like "I performed badly", and also usually refers to a contest or fight. In simple terms, "I can not believe that I was so badly beaten". To me, "come off it" has more of a "stop being dishonest" meaning. It is suggesting that you don't believe what the person is saying.

Gary gave an excellent answer. Here is an excerpt from the Oxford Learner's Dictionaries website. come off - phrasal verb come off well, badly, etc. (informal) to be successful/not successful in a fight, contest, etc. I thought they came off very well in the debate. source: https://www.oxfordlearnersdictionaries.com/definition/english/come-off?q=come+off

It's an intended result or an effect. Here it means "I cannot believe it happened like this or this badly"

This has at least a couple of possible meanings. To come off badly might mean to do badly in terms of money or reputation as a result of some action or other. It can also be used to mean to seem, to appear - "to come off badly" can mean to give a bad impression. Merriam-Webster has : Definition of come off intransitive verb 1a : to acquit oneself : fare came off well in the contest b : appear, seem 2 : succeed a television series that never came off — TV Guide 3 : happen, occur 4 US, informal —used in phrases like where do you come off? to express anger or annoyance at what someone has said or doneMarie was bent over the table now, reading, her piping girlish voice hot with indignation. "Where does she come off, anyway?"— T. Coraghessan Boyle transitive verb 1 : to have recently completed or recovered from coming off a good year 2 : to have recently stopped using (an illegal drug) an addict who is coming off heroin at https://www.merriam-webster.com/dictionary/come%20off