Forget about embouchure, strength, flexibility and scales, if you don’t get the breathing in place, nothing else does´t matter. You probably know about a good-lip-day. I would say that 9,3 out of 10 times, it is actually a good-breathing-day. If you make sure airflow is relaxed and controlled, your lips will have perfect working conditions.
Have you tried not playing for a while, maybe a week or so? And then picking up the horn and to your surprise it is actually responding quite well. This is because you have had some time to loose some bad breathing habits, and play more relaxed than you usually do. Hold on to that feeling! (Problem is you will only play well for 5 minutes, since you lost a lot of strength…)
This simple, fast and fun (yeah, right) exercise will help you gain control over your breathing and improve free flow.
1. Find a metronome. Any model will do as long as it tics/pings/rattles or blinks.
2. Put it on 60 bpm
3. breath IN for one bar (four counts)
4. breath OUT for one bar
5. breath IN for one bar
6. breath OUT for two bars
7. breath IN for one bar
8. breath OUT for three bars
9. breath IN for one bar
10. breath OUT for four bars
I think you get the picture by now. See how many bars you can exhale (reasonably) controlled. 4? 7? 9? Try to make sure that you get all air out evenly when you exhale. That means that exhaling for ones bar is like playing a f , while exhaling for 8 bars is like a pppppp.
Variation: try it in 3/4 instead of 4/4.
Now do the same thing on the trombone. You might start at step 6, since it is hard to loose all your air on four beats. Go for a comfortable medium/low range tone, somewhere between F and Bb. You can change pitch for each note you play.
I recommend that you do these exercises daily for a period, and then let me know if it was helpful, comment section is open.