OP- dont beat yourself up over this stuff, all good things take time. You know what they use to say about getting to carnegie hall right ? practice practice practice ! I dont know what sources your using to try and play these tunes, but I,ve found that just listening to the material without bass in hand, absorbing the vibe if you will, helps with the feel the original player is coveying. I learned sir duke when it came out as my band at the time played this and others from that great album ( I still say album, Im old) Funny though, to this day I can still rip off that passage in sir duke note perfect even after 30 some years, though I havent played the song since the late 70's. Some stuff just sticks with you I guess.
"I want you back" is a great feel tune, the break down part gave me fits till I relaxed and just listened to it a few times, once I got it in my head it fell right into place. Good luck, just stick with it and you'll be fine.