I define expression as any sort of communicational means for advancing one's intentions. Or something. A baby cries to express hunger and/or other needs. Then there's artistic expression, which is basically the same, but through artistic means. Whatever art is. Musical expression is the same, but through musical means. For a DJ, selecting a song is musical expression. For a guitarist, things that can be done with the guitar are expression - which is a LOT of different things on many different levels. It doesn't have to be just about how to pick the strings right - picking the right piece for the situation can be within a guitarist's means too, if you like to see it that way.
You have a palette of things you can do, and you have some freedom for making choices, within restrictions and limitations. What are the things you can do technically, what is appropriate to do, what is suitable for the genre etc. Do you remember the name of the song or can you look it up? Do you remember how a specific trick is done? Even if you remember, will you be able to carry it out technically? Technical and theoretical exercises can give you more freedom by expanding your physical and intellectual abilities. Practicing pieces gives you freedom, because it increases your confidence in selecting those pieces. Etc.
It's possible to perform music completely mechanically or according to cultural conventions. "We always play this song in this situation, and we play it exactly like this." Ok. That can happen, and someone walking into that situation for the first time can be very deeply touched by the music, but I wouldn't say that the musicians expressed themselves very much. Expression should be about making choices.
But even if you have unlimited technical, intellectual and cultural freedom and confidence, how do you choose what to do? You should have some kind of an idea about what will happen if you select each particular song, chord, note, dynamic level, technique, type of vibrato, etc. The ability to predict the outcomes comes with experience, and to build experience you of course have to go through enough repetitions in practice, but you also have to have certain sensitivity to the effects of your actions. If you're deaf, you can't hear the sounds you're making, so you lack sensitivity - you might be able to see the effects from other people's reactions though, but I think that will be very difficult. Like, does the teacher say "good" or "not good"... Sometimes music education seems a bit like that! Deaf people trying to play instruments, looking at the teacher's reactions to know if they're doing it right or wrong. I think that's a slightly mistaken idea. Music should be about hearing, listening, feeling. Why are people so obsessed with theoretical descriptions of harmony, "M7 gravitates toward root", stuff like that. Have you ever dropped a ball - which way did it gravitate, toward the ground or away from it? Theory and teachers shouldn't be needed to introduce this basic phenomenon to students, or to confirm and validate the observation, only to give a name for the phenomenon. But unfortunately, sometimes the students haven't had a chance to play enough, like with toys, drop things and throw them around. In my opinion, such toying should be encouraged, to make sure that the students have personally encountered the phenomena in practice.
How to learn to express yourself through different elements of music? By practicing, listening and feeling. If you can't hear, if you can't feel any difference in different ways of playing notes, articulations, chords, phrase lengths, etc. that's going to be a problem. Whatever the thing is you want to utilize as your expressive tool, you have to practice doing that, with open ears. Teachers and lessons can provide you with new ideas to try and certain aspects of music to pay special attention to, and then you take those new ideas and start practicing them and focusing on the suggested aspects to get a feel for how they change things. If you can't technically perform them, or if you can't feel any difference even after trying the ideas for a long time in various situations and pieces, then maybe that's just something you can't use for expressing yourself. Find other ways of self-expression, there are many. Everyone can find some level and area of musical self-expression, and that defines them as a musician. Some people are content with selecting the right Spotify playlist, and that's perfectly ok, that's their level of musical self-expression. Some people might not notice any difference between any Spotify playlist or none at all, so then that's not their thing obviously. Though maybe if they're motivated, they can develop a sensitivity for it through persistent listening.
If you have trouble noticing the right aspects in the music you're making, a teacher could help you by giving ideas and feedback. "Your phrases are too long - can you try making them about the size of a line of lyrics, with some punctuation in between? Something you could actually speak or sing as a continuous phrase." And then, you take this new idea as part of your practicing, and perhaps you start paying attention to the lengths of phrases in music you listen to.
To sum up: you learn to express yourself by repeating expressions and sensing what effects the expressions have. Whatever level of abstraction the expressions are on.