"To quit" is an irregular verb in English, the past tense is just quit instead of quitted. So "I quit" can be either present or past tense, but from context it would be clear that "I quit infosec" is past tense.
Grammarly helps but having been in infosec, you probably will have concerns about sending your private data to that cloud :)
"I quit infosec and I couldn't be happier" is how I'd have written it.