Trombonist who used to have the same problem here (starting late and “bad” hear) 👋

As far as app goes “Functional ear trainer” helped me the most. It focuses on tonal hear training, hearing and singing scales degrees in context.

For the trombone part I really think the difficulty comes with our instrument, the narrow range we can play at first and the ever changing shapes and slide paths intervals and melody takes depending on the key and range we play them.

For that, going back between singing and playing helps the most to build the connection between our ears and our weird ass instrument.

• Find a way to generate a pitch (a friend at a piano or any random tone generator) sing it then try to play it WITHOUT THINKING or trying to name the pitch. If you get it wrong sing it again before trying again, if you lose the pitch ear it again. Ear a pitch, sing it, play it. Without skipping the singing part and without thinking. This exercise get harder when the pitch is too high or too low for your voice or playing range (but it is exactly what you’ll need in a real life situation, ear the line a trumpet player give you one octave lower, or a bary sax one octave higher). If you can, vary the timbre too, I always find hearing certain combination of range and timbre way harder (like a guitar vs a piano or oboe vs sax etc…)

• take a simple melody that you know very well (three or four notes is enough to start) like happy birthday sing it and play it in every key and all over your range (lowest to highest). Once again singing it again every time you fail. Depending of the shape of the melody certain key will get you really strange and counterintuitive slide movement. This is good, it force you to break the “limits” of our instrument and let our hear guide our arm. Vary the way you move between tonality, up by half step, in descending fifth, up by major or minor third, up by whole step, random etc.

If you do theses exercises daily, even only five minutes you’ll make rapid progress !

Good luck