For those of us who write, rejection is constant. It takes some getting used to. Some folks give up before they get really get started because of the rejection.
When you put so much of yourself into something a rejection of the thing feels like a rejection of you. In that case, the first rejection letter confirms the terrible voice always whispering, you’re not cut out for this.
So they quit.
Never mind that Harry Potter was rejected a dozen times. Or that Stephen King impaled his rejection slips on a spike when a nail couldn’t hold the weight.
I’ve seen one too many good writers give up after a couple of rejections. That’s why I turn my rejections into poems to inspire other people going through the process to keep going.

The only way forward is to keep writing and keep submitting your work to publishers. There’s no shortcut.