I think the thing with rejection is that individual ones don't mean much, but patterns are always concerning. One query letter rejection, whatever. But you send out 20 and get no requests, that might be a query letter that needs work. If you get some feedback that says "The book has promise, but the opening is too slow," eh, that could be one person's opinion. If three agents tell you that, it definitely might be too slow.
I remember when I sent out my first queries (to people I found by googling "ten best agents in terms of sales", very strategic) and it was 50/50 silence and form rejections. Well, I realized my query not only was bad, but it didn't even make sense. So I reworked it, sent it out to some more, and got a few partial requests. That didn't seem great, so I reworked it again and started getting tons of full requests. I was really glad I took those breaks to re-work it because otherwise I would've kept sending out the same bad query and burning through names.
I try to stay positive by reminding myself that if you learn from your mistakes, they weren't really mistakes in the big picture. So those projects that "fail", whether they're books that just never come together or don't sell or whatever "success" means to you, you've learned something and grown and hopefully gotten a bit better. Most writers don't sell the first thing they write, after all, so maybe you need a few projects to figure things out. And even after they sell something, they don't sell everything else they write either, so maybe you need to keep figuring things out.