There are many aspects to being a full-time professional writer that take some getting used to...long hours with a sleeping (sometimes farting) cat in your lap whose paws manage to find keys you never knew existed on your keyboard ... or microwaving the third cup of coffee because there are no other office workers to run the endless cycle of the machine. You just have to adapt and these things get easier.
One part that doesn't ever really get easier is having a project rejected. But you still have to get used to it, because every writer (really every creative professional) gets rejected ... a lot. It's part of the process. That doesn't mean it feels any less like a punch in the gut each time it happens.
It's never as bad as the illustration I did to show my point. It never comes back with a bold "You're Stinky" on it (well, not most of the time anyway). But no matter how kind the rejection, basically it's the same as a six year old sticking their tongue out at you and telling you that you are a stinky monster. It always feels personal. That's because as the author, you invest so much personally in the characters who you're trying to find a home for within the pages of a book. Being rejected is basically someone saying your characters are not welcome here.
That's how it feels anyway.
The reality is, publishing is not only about coming up with a great entertaining story, it's also about finding the right partner to pair it with. There's a lot of trial an error there. (Which is where an agent helps). This is the fact that I always keep in the back of my mind. It's what helps me lick my wounds and get up again the next day and try, try, again. After all, these characters need a home.