After reading your posts, it appears to me that you are probably trying to do something too complex. If you want to learn Paul's Bass in Penny Lane, then getting a transcription of that would really help and start learning note for note. You can also take an MP3 of that song and put it into any slow downer type software and slow it way down without changing the pitch and can segment off small parts to loop and play along with. VLC player will do this for free as well as Audacity, BUT my favorite is the program Transcribe! (with the exclamation point) I use it constantly. It even slows down instructional videos.

With BIAB however, I think maybe your backing tracks should start out super easy. I always start with 1 chord only looping to practice new scales, triads, roots, pentatonics etc. Then you can expand to other single chords. Then you can expand to 2 chords back and forth with enough bars of each that it is not too fast for you. Then you can slowly get more complex for instance building a backing track of I to IV to V to I and play over that. Go slow and incrementally. Or you can take two chords out of the Penny Lane progression and play your newly learning bass run over just that segment. You could use the style picker song search box to find something suitable similar to Penny lane. Or just go back to the real song and make segments to practice against if you want precisely the Beatles music.

Have a ball!

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