Oh, don't we groan when those rejections come in? Really, is there any more soul destroying moment than answering the door to the postman's knock only to find him standing outside with one of those ominously familiar packages in his hand? It's back. Oh, no. Please let me hide under the bed and scream and scream.
As the rejected novels pile up in a corner, we alternate between mood swings of hope and despair. Can't I ever get a break? What is so hard to believe is that you are getting a break. You're being forced to write and write and write. Sooner or later you are going to get it right.
The point is you can't help but get better at it. You're working your writing muscle. I've recently had to start the most ghastly painful exercise class to build up core muscles to support my dicky back. The first time I knew there was no way in the universe I was ever going to be able to do some of the stuff the tutor was asking me to do. What do you know? A few weeks in, it's still hellish painful, but I can do most of it.
The tutor tells me it always will be painful. So is writing at times. We've all experienced the fight to stop displacing and get on with it, or writing through days when your brain is made of mush. But you do it, and you get better at it just by doing it.
I've got more unpublished novels lying around than I care to think about. But if I didn't have them, I wouldn't be writing as well as I do now. What's more, I've got a mine of developed stories to recycle at need.
You're not wasting anything. You're learning all the time. Keep doing it. Someday someone will start paying you to do it. And believe me, that ain't no picnic either!