Writers hear 'no' far more often than they ever hear 'yes' over the course of a career. You'll go through spells when everything you touch is finger-lickin' good, and other spells when every pitch, spec and proposal gets dismissed as arse-wipin' bad. Of course, there are many kinds of rejection. The straight dismissal with no explanation or chance of feedback - that's a pisser. The constructive criticism rejection, that's at least helpful.
Then there's the 'great idea, we've already got one' rejection. It's a compliment of sorts but no less frustrating. Yes, you're coming up with ideas that match what people are looking for, you have your finger on their pulse. [Of course, it could just be unhappy coincidence, but that's the bad voices at the back of your psyche talking.] You feel like your pitch might have gotten the nod, if you'd just submitted it sooner. So, a bit frustrating.
Rejections are one of the reasons I don't have pets. Why make some poor animal suffer as a surrogate for my frustration. So your old ideas got rejected? Get over it, move on. Better yet, have a fistful of fresh ideas ready to fling into the machine. Experience is what you get when you didn't get what you wanted, as a wise man once said. Persistence plus talent always wins through in my experience. So this is me, keeping going. Onwards.