I learned to play guitar as a kid, played some classical and jazz as a teenager and in my twenties, then put it down. I have a nice classical guitar sitting in my closet for a decade.
My problem is that although I can read music, understand some theory, can play chords and even some can play a little bit of complex music like Villa Lobos, I have absolutely no "ear". I can't even tune the guitar without help.
So this means I basically need to memorize or play from sheet music, which kind of got boring after a while.
You don't really need a good ear for tuning. Pick up a tuner, they're pennies nowadays.
When you're tuning by "ear", though, the trick is, on an acoustic guitar at least, to feel the body resonating. It's much more a feeling than a particular sound. I think very few people can just pluck an E note out of nowhere. That takes years of practice.
A general "ear" for melody and harmony comes from practice, exposure to different music styles, and playing with other people. Some stuff that sounds awful by yourself sounds much better with accompanying instruments.
I'd highly recommend the "singing against a drone" exercises in "the harmonic experience" by W.A. Mathieu. You can do them with an acoustic guitar, and they will improve your ear immensely. The first exercise is simple: singing exactly in tune with an open string.
Try singing the melodies while playing. I never really wanted to be a "singer" but found that practicing voice has improved my ear and more intuitive side of the instrument.
My problem is that although I can read music, understand some theory, can play chords and even some can play a little bit of complex music like Villa Lobos, I have absolutely no "ear". I can't even tune the guitar without help.
So this means I basically need to memorize or play from sheet music, which kind of got boring after a while.