I have friends that, from the first time they pick up a guitar or bass, can listen to a song once and play it exactly the way it is written. Hate them but in a good way ;)

If you are trying to learn a lead that someone else wrote and you are like me (from what you wrote you are) then you will just have to keep at it over and over. Start off slow and when you have it nailed increase the tempo. It may take you a week or even months but it will come over time.

I tend not to worry about playing a song, be it lead or rythm, exactly as it sounds. I just get the basics and then add my own "flair" to it since I have no need to be precise. Still sounds good when played with the accompaniment of others and you can tell what I am playing, most people hear a difference but can not tell what exactly that difference is.

If you get to frustrated with it then just make up your own lead for it :p

MSF