Stick to contsant picking on one string, wen you've mastered 1-2-3, 1-2-3-2-1, 1-2-3-4, & 1-2-3-4-3-2-1 then try adding the next string in. The only way to get this is by practice. One of these days the timing of the right and left hand movements will just click into place and you'll never have a problem with it again. Just one of those things, practice makes perfect.