Wanting to get better at music, how should I attack it?

So, I started music with drums, so I have a good understanding of
rhythm, but I've been playing guitar and piano for a while now, and
I'm really bad at reading notes well and learning anything harmonic by
ear.

So I'm looking to get better at music as a whole, how should I do
that? I'm hoping to be better at guitar than piano, but would it be
better to just focus on piano because it's more laid out and easier to
understand? Or should I dive into music theory, which I have been
putting some time into lately, and if so what should I look for in
theory?