.. P, when i first started playing i struggled just like yourself.. first off why did you give up lessons? a GOOD teacher is invaluable!!! i cant stress that enough..
maybe you didnt find the right teacher.. there is nothing like the one on one.... i had piano lessons for a little while and its so important to have that close information contact.. i taught myself how to play bass and i think it shows in my theory, and i think that really thats why i wanted to play in the first place.. dont play theory catch up like im doing...if that what you want from the instrument?
i usually find Youtube information to be bit by bit.. you never really find the info you want....unless Jack's suggestion might be good mind... if you dont want to go back to a teacher i recommend a book by a guy called Stephan Richter, its just called "BASS".. comes with 2 CD's.. its THE most comprehensive book ive ever come across, and ive seen lots... if youve got the time and patience its all there..
of course a book like this will bring up questions, so i go back to my first point.. a good teacher.. no matter where you go you will always want to ask questions about stuff you may get stuck on... the best way to learn is from human beings who can can show you in real time.... from a book it depends how well you soak up information and understand it.
also listen to music.. im sure you do. but really listen, and listen all the time... i used to surround everywhere i went with music and still do
there are much more knowledgeable guys on here, that could prob give you some better advice.. i dont know them, but maybe you could ask guys like Bilbo, Doddy.. i dont want to speak for them, but im sure they wouldnt mind a quick Q..
i know what its like to need answers to something your passionate about...
i wish you well