Where to begin? As the Irish joke goes - "I wouldn't be starting from here."
L

L!
Im rather like you, notsunshine...play more by ear than reading off scores, prefer to listen and know the melody before i attempt to read the score, learn off scores the parts i like and forget the rest or make up my own version, learnt by myself for a while (tho not 15 years!) did all the dilly dallying, bits and peices here and there...and then, HIT a BRICK wall, where i didnt know what to learn anymore or how to improve, AND found myself here asking questions too

As im more 'improvise' focused, which i think u may be too, reading off sheet music is EXTREMELY TEDIOUS and hard work for me! and i knew that if i were to improve, one sure way of improving is learning pieces i like and then improvising in that same style BUT i knew i wouldnt really be motivated to do it myself, by myself...so i hunted around for a good teacher who was able to improvise as well as teach regular piano, tried a few with no success but FINALLY found one i really liked!
so now i learn a new piece every week, memorized totally, have a lesson once a week and my teacher will really show me how to add dynamics and interpretation into my piano playing which i didnt even notice i lacked to begin with!! and i find myself using those same techniques and styles in my own improv!
IM also working from improv books by myself and theory but having a teacher and another PERspective REALLY goes hand in hand for me!
of course, were all individuals, so what works for me might not work for you, but just thought id share

Being able to improvise is good, being able to read music is good, being able to do both is better

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All the best!
Estelle.