#1: Signed with first agent after several books on query, took 9 months for her to offer and she offered when I nudged her with a mid-size pub book deal. She then dumped me one year later because she "no longer loved my voice enough to champion it the way it deserved" after subbing book #2 and not selling. We turned it into a YA. She literally went from one email exclaiming how excited she was to sub it soon as a YA to one week later, a break-up email with no further explanation. I insisted on a phone call and she was very distant. Funny how she really liked me when I already had a deal in hand...
#2: After a few days of feeling sorry for myself, I subbed that YA and within 3 months got 2 offers. That book did not sell (well, actually it's going to acquisitions now, but that agent did not sell it...okay, actually she supposedly got an offer from a Big Five that went silent for months then retracted the offer?? Not sure how often that happens, but it sounded so sketchy.) She subbed two more books (that she'd neither give me the sub list or responses for or even told me that she'd gone on sub). I went up and down about leaving for six months even when everyone told me the relationship was not right and agents should not ignore emails and phone calls and keep editorial/sub info from you, etc. I accepted that having no agent was better than having a bad, neglectful one who obviously did not respect me. I left just short of 2 years after signing with her.
#3: I went into depression and stopped writing for 10 months and traveled the world. I returned with passion and said screw it, I'ma write because I want to. Finished that book in a month, queried Jan 2nd of this year and received 3 offers in 3 weeks. I signed with an agent that I knew of because she's an agent at a good friend's agency and has been there forever and her clients all speak highly of her. All the offers were great and it was a difficult decision, but this time around in querying, I only had a handful I went after because I wanted serious agents, ones with good reps, good sales, happy clients, and known for good communication. I adore her and her notes and how she keeps me in the loop. I don't even have to ask her and she tells me btw, I'm going to be gone at BEA or on vacation or whatever during these dates, or let's discuss potential editors for sub lists and why one would work better than another, etc. And she gives me dates and game-plans! And forwards editor responses! I have absolutely no worries with her and when we get passes from editors, it's totally not a big deal because I trust her.
What would I tell my querying self? Seriously, no agent is better than a bad one: Go after ones that you really want and who have exceptional track records with their happy clients, not just anyone. See how they interact with their clients and editors. It's fine not to get an offer for a book because all those shelved books made me carve out my craft and grow as a writer. And once you land an agent, speak up. Don't fall for that "they're busy" or "I'm bothering them" bs or constantly give them the benefit of a doubt. Yes, be professional and patient and forgiving and don't nag at all hours of the day, but if you want answers, you ask and if you don't get them for BS reasons, then you walk. You deserve a good agent if you are a good client.

And be happy. Rejections pass, just write and learn and write some more and be happy with your work. Do not stress.