Last night and this morning, I gave some thought to the Janice Lester question.
Memory Alpha article aside, I don't recall any canonical reference to Lester actually serving in Starfleet. Can anybody else cite anything?
Janice refers to "the year we were together at Starfleet." So she was attached to Starfleet in some capacity for a period of at least one year. I've always assumed she meant Starfleet Academy, that they were together there but she either washed out due to her instability or, as you suggest, quit due to being deemed unsuitable for command training.