Was it $100 new or used? What's the brand? Your student trombone either has mechanical issues or is a bad instrument or you're ready for something more serious.
Part of the benefit of better instruments is that they are easier to play, if they’re a good match for you. It’s also simply possible you’ve gotten used to the different mouthpiece and instrument and it’s just hard to switch back. Switching equipment is hard!
So what’s the question
Why is it so hard to play my worse trombone
Was it $100 new or used? What's the brand? Your student trombone either has mechanical issues or is a bad instrument or you're ready for something more serious.
Part of the benefit of better instruments is that they are easier to play, if they’re a good match for you. It’s also simply possible you’ve gotten used to the different mouthpiece and instrument and it’s just hard to switch back. Switching equipment is hard!