As hard as it might seem, rejections are a good thing. It means that your emails are getting read and which agents will reply to you by email for the next manuscript that you have to pitch when you have more knowledge on what to query them.
Another bright side is the that the more variety you have to approaching your query and submissions, you can see what’s more likely to hook people. This is trial and error and you can’t improve your next run of trials without errors. The further you get into the process, the more devastating the rejection will be but at least you still had the excitement of getting a request. Some writers struggle to get even that.
My eleventh piece of advice: collect your rejections in the name of research. Submitting is a process, if you get it first go, good on you, but you’re in the minority. Most of you will need to go back an forth trying different titles and angles to sell your book. Ganbatte!