So we've been practicing and you're seeing progress. You've started an exciting journey to becoming a better musician. But you've hit a road block, you've plateaued. Thats just fine! Don't worry, this happens to everyone (even the folks that don't seem to ever stop improving, they just aren't talking about it.) Plateauing is a natural state (shoot, we've named geological structures this so it has to be natural.) I know in todays culture we should feel like we're always going, always making progress, always doing something. But guess what, I'm gonna go ahead and call BS on that lifestyle approach. And here's why!
I'll start this off saying that I am not a philosophy professor, or some self styled millennial guru out to sell you on meditation and yoga (they don't hurt though.) Plateauing happens to us all. Sometimes after making great breakthroughs, it is nice to rest on our laurels and reflect. Other times the plateau can be very inconvenient, occurring as you work towards a breakthrough. This is the plateau many of us are familiar with and find very frustrating. The reason we practice daily is to continue improvement. Sometimes we just hit road blocks that take time to work through. You just gotta bootstrap your way through these plateaus.
There are all sorts of reasons these plateaus happen. There's the physical side, maybe your air isn't cooperating, or your slide arm isn't as accurate as you want it (our ears develop at light speed in relation to our actual playing. We can identify what is good and desirable way before we're able to execute it on the horn, leading to lots of frustrations.) There is a mental side, are you getting enough sleep? Are you exercising at all? Are you eating a balanced diet? Is there external pressures to perform? Are there self imposed internal pressures? How you choose to handle these is up to you. But you will face these issues, and while many of us don't like having to face our weaknesses, turn this into an opportunity to making improving yourself a positive trait and not something you avoid and/or don't enjoy.
In the end, cherish the moment of the plateau, no matter its (in)convenience. Recognizing what is occurring is important to taking that next step forward. It could be a couple days, it could be a week, it could be a month. What you cannot do is let the plateau discourage practicing (it will!) You must persevere through this challenge. Again, this is a moment that defines those who are willing to do the work themselves instead of waiting for someone else to do it for them (i.e. you're teacher meticulously trouble shooting the problem over several lessons.) Use this as an opportunity to work on fundamentals (or if your fundamentals are what's plateauing, find some simple etudes to work on musicality.) Find a way to refocus your attention elsewhere in your playing and tackle something off of your "if I only had more time I'd do this" list. It may not even be trombone. Maybe you go clean your room/apartment. Maybe its to get some exercise in. What I'm trying to say is: don't dwell on the plateau. Stagnation in one area does not mean you're a failure at all areas. Go work on something else, and come back to the plateau another day. Refocusing your attention sometimes is all you needed in the first place, and the mental block will go away.
I wish you all the best luck in handling this issue in your playing and life. Its easy to get hung up on what isn't working. Don't forget to take advantage of that to reflect upon all the good deeds and work that you've done leading up to the challenge. Our path to our life goals is different from those around us, and we should not compare the success of others as a failure of our own.