I have a repertoire of songs that I picked up, rather than learned, from records and the radio in the 60s. I would say that I started teaching myself to play guitar in 72, but I have no difficulty playing those songs because they are so familiar. I would say that it is one thing to learn fancy picking, but this is a matter of practice and dexterity. The other areas to focus on are the ear and the memory. How you can improve the memory for songs I don't know. Playing by ear is a gift that can be improved by the techniques used in helping people to sing, especially in improving pitch. To expand this, the best musicians recognise what they hear as something they can play - particularly, they recognise the sound of chords and arpeggios as something they have played (every chord on the guitar is an arpeggio - even if strummed fast, the strum goes across, and every shape has its sound). If you learn to play from paper, you don't absorb the tune - you play the dots or tab, and the tune comes out. Take the paper away and you can't play the tune. I learned the piano when young. Put the dots in front of me and I'll play Cry Me A River on the piano. Take them away and I can't do it. Give me a GUITAR and I'll play it in all thirteen keys, but don't ask me to sight-read the melody on the guitar - that takes a fortnight's practice at sight-reading. When you have learned those fancy picks and tunes, you have to listen to other tunes you haven't learned, relate the musical patterns to what you should know, and pick them out without help from teacher or paper. That's when you start to LEARN the songs.