I think it’s normal for anyone to feel that way when starting a new instrument. I also think sometimes people (not necessarily you) assume it’s easier to sound nice on a whistle than other instruments, when really it’s no different, with the same effort required. I felt the same way as you when I started on wind instruments - very discouraged at first. Just give it plenty of practice, and you’ll be very surprised by the progress you’ve made in one year.
I wouldn’t say it’s necessarily not “your instrument” , but it simply requires something much different than percussion does, so it might be a tad outside of your musical comfort zone, and requiring a different approach than you’ve taken to drumming. It might be best to just try to forget about any preconceptions you have about learning instruments, which you have acquired from your drumming experience. It’s apples and oranges. While there are some similarities in learning any instrument, the demands of whistle and percussion are too different to judge your learning of one, based upon the experience with the other.
When I started on wind instruments, I wanted to snap it in half and just throw it in the trash everytime I played it during the first quite a few months of playing. I thought it would be impossible. You’ve only been at it for a matter of weeks, so give it time and I promise you’ll get past that phase of it seeming impossible.
And yes, as mentioned, get as much exposure to good trad whistlers, and whistlers in general. After you have plenty of exposure, find whistlers whose sound appeals to you, then pay close attention to the details of what they are doing to get that sound. Of course, it doesn’t hurt to learn music you are already familiar with in the beginning, either. No reason one can’t do both.
I will say, it will only discourage you if you are measuring your progress on a week-to-week basis. Every few months look back at the progress you’ve made, and then you will notice it’s more significant than you think. After a few weeks or months after starting, though, I wouldn’t expect anyone to be sounding good playing anything. That’s absolutely normal. Prepare to sound terrible for quite some time, but just remember, it’s temporary.
I’m by no means a professional, however I was once in your situation , so this advice is based upon what’s worked in helping me to steadily progress.
Once you hit a milestone of learning to do something on the whistle that you thought you couldn’t do, you’ll be so invigorated by it that you probably won’t want to put it down. It takes some time to reach that experience after starting for many people, though.
Good luck!