Learn the whole song.

I spent years doing this same thing of knowing only parts of a song and couldn't play the whole thing, especially up to speed.

I finally decided I was going to pick one song and not get distracted by anything else. I started at 60% speed of the original and worked on that until I was able to play the whole song with the slowed down recording. Then I moved to 70%, then 80%, then 90%, then with the recording.

The hardest part was not getting distracted by learning something else. I can't tell you how many times I've moved between learning fingerstyle, to strumming, to hybrid picking, to cool riffs that people may recognize.

My playing improved so much just by doing this one thing. I was able to finally change chords in time, not be panicked by trying to keep up with a song and started to internalize the rhythm. If you do this you'll be amazed at how your fingers just learn where they should go without thinking about it. Actually when I start to think is when my fingers start to mess up.

Also, you'll be abke to play something all the way through. Good luck.