Getting rejected is no fun. It’s part of the writing game, though, and so instead of complaining about it, it’s a good time to take stock.
If you are lucky enough to get a personalized rejection, you at least have some feedback to consider. Whether the agent didn’t connect with your work, or if they’re already repping a similar manuscript, you can take that for what it’s worth and continue on.
A form letter is a different animal (and don’t think that they don’t happen with partials/fulls). You have no idea why your work has been rejected, what might be wrong with it, or how to fix it. You can guess, but should you go to the trouble of rewriting, or just assume that you haven’t hit the right agent yet?
Critique partners and beta readers can be helpful, to an extent, but in the end, you are still the one who decides whether their suggestions are good ones or not. If you are getting the same comments over and over, like, “it doesn’t flow well,” or, “your main character is unlikable,” it’s time to pay attention and do some editing. Otherwise, just keep an open mind, and keep researching agents/sending query letters.
Sooner or later, you’ll hit the jackpot, or you’ll write something better, or you’ll crawl into a literary hole and decide that maybe writing wasn’t the life for you, anyway.
However it works out, it will.