You spend a lot of your time working on stuff that will never be seen. How do you deal with that? How do you internalize that?

I'd say you have to focus on the fact that you're getting paid for it, at least.

Jokes apart, I would say it's expected. If all your proposals are stuff you like, you will not be disappointed by any choice.

If everything you've made is pretty solid, you can be satisfied just in the fact you've made some pretty cool stuff, even if no one really gets to see it. At least you get personal satisfaction out of it, and maybe they'll be used for other stuff later down the line, even if they're not used for the original purpose. So you make peace with it, I think.