It is common to experience what seem like setbacks even when playing just one instrument.
Often thiis merely means that an early easier-but-more-limited habit, such as crushing the lips with mouthpiece pressure for high notes, is being replaced by a more-versatile improved new technique, such as developing balanced muscle tone throughout the lips.
Each improvement will progress from being awkwardly unfamiliar to eventually being another dependable habit to progress beyond, again with a temporary setback.
Switching between trumpet and trombone is a perfect time to learn
more about the huge role that your vocal tract plays on any wind instrument.
I consider my euphonium and trombone to be only the visible halves of the whole instruments.
If you haven't already, try alternating between playing some high notes and just singing them.
If you can whistle, try that too.
Singing and whistling train your mouth, tongue and throat to resonate each note just as much as your horn does.
Then pretend that you are singing, or whistling, while playing.
You will likely not only hit high notes with less effort,
on both instruments, but also realize that you can more deliberately
color your tone
with sweetness, mystery, punch, swing, twang, and plain old brass razz,
along with other moods and special effects.