The only reason I didn't cry in the bathtub when I got rejected by acquisitions (followed by several more editors who basically said they love it and they know acquisitions will reject it) is because I'd just been dumped by my now ex-fiance. Looking back, I'm kinda glad the two coincided because there was only so sad I could feel. I guess good advice for coping with publishing rejection is to also be coping with romantic rejection, though I don't exactly recommend it haha. A year later, I'm in a much better place with both my writing and my life, so there is light ahead, but I totally feel you re: bathtub and crying.
Good luck with the evaluation, pinkbowvintage!
The red flags for leaving an agent during submissions would be, 1. poor to no communication (won't send you the sub list, goes radio silent for months, vanishes off the planet), 2. evidence that your agent can't get editors to read your manuscript (if your agent won't tell you who is reading it, if you know the pitch list and then a small percentage of that pitch list seems to be reading, if no editors seem to have your manuscript at all), 3. rejections that suggest your agent was waaayyy off base placing your MS with that particular editor (all simple forms, "just not for me," "had a hard time connecting to the story," "not quite right," with no details, from every editor. One or two of these is okay, but if every rejection is like that, no good), and 4. if your agent is only pitching to small houses/places that accept unsolicited, unagented submissions (aka your agent isn't doing anything you couldn't do yourself).
Yes, there are agents who are out to drinks with editors every night in NYC, and agents who have never met face to face with an editor, but from where I'm standing, if your agent is getting your manuscript in front of Big 5 editors, and getting nice/detailed responses implying a good relationship between agent and editor, I'm not sure how much changing agents, even to a heavy-hitter, will make editors not reject your book. Plus, if a bunch of editors do have your book, pulling out with your agent and re-querying could be complicated. The times to get out are either when you realize no editors or only small editors have read your book, or if you have written a new book that you could query new agents with. Another good reason would be if your first and second book were radically different. The agent that pitched your MG about fluffy bunnies might have no idea where to send your splatter punk gorefest, and may not want to try.
There's also always a possibility that your agent just doesn't know what good or sell-able writing is, and that their love for your book is meaningless because it's not what editors love. This is my nightmare, and I try not to think too hard about it. That I could have deluded my agent and all my beta readers and myself into thinking my book is great when really it's just okay. Successful agents make a living by knowing the difference between a book they love and a book they can sell (hopefully ours are both!). If your agent isn't making any other sales, if all your rejections are lukewarm, if you have a really bad feeling in the pit of your stomach, then it's time to amicably part ways.
Man, I bummed myself out. Still don't know who has requested from our pitch, which makes me extra nervous because I wrote the bulk of the pitch (my agent and I edited it together), and though I trust her to tell me if my pitch was shit, I'm nervous nobody will request this really lovely book all because my pitch wasn't right. I'll probably hear from my agent tomorrow, since that's our day to check in, and I've emailed her about four different little things this week unrelated to this sub. I have so much hope right now, but it's all tangled up in knowing I had this much hope the last time, and it all got squished.
Edited to add: JoyMC, that sounds like an agent worth staying with, as long as you still feel good about it.