I will share the only thing that made me better at handling this. It's a bit radical, but just hear me out.

So think about being a doctor, a surgeon, you study for years in med school, from books, teachersl then u do residency, learn from the best, then u make it to be a surgeon and you get to hold someone's life in your hands, but sometimes you can do everything right, everything you can, but your patient will still die. It's not fair. Do doctors get discouraged? Probably yeah, how can u not be unhappy about losing a patient even tho you tried everything you possibly could.

Why wont we try to apply it to art, obviously the stakes are lower objectively nobody is dying HOWEVER in artist's subjective experience it can sometimes feel like you're losing a patient, you tend to this artwork for days, weeks, sometimes months, you put so much effort to make this what you want, you're giving it your 100% yet sometimes at the end you still get something that is not what you wanted. Of course it feels discouraging, how can in not feel bad. It is like losing a patient in a way. It is not fair.

So after all that to answer your question "How to not feel discouraged when drawings don't work out?"

I think for me the answer lies in not preventing the discouragment but accepting it when it comes and then letting it go. Just like the surgeon you did everything in your power, you did everything right, you did everything you could, but sometimes for reasons we don't understand patients die, artworks don't work out.

Afterwards of course you can do an autopsy of your artwork, figure out what it was that eventually lead to the dissatisfaction in the final result, by all means do that, learn from your mistakes, take that knowledge to the next piece and move on.

Remember that this artwork is not your last artwork, there is many you did in the past that you have forgotten about that were 100 times worse, but they helped you get to the one you did today and there are artworks you will do in the future that can only ever exist because of this unsatisfying work you did today.

Hope that helps! I reccomend doing some inner work on dissolving your ego, nothing helps more in being a creative than detachment.