Try following the melody. If u already have the song in yr heart, then it`s always easy to speed up or slow down. Caz everything will still retain its rhythmic structure, but perhaps more stretched out or compact than b4. But u must know yr notes extremely well b4 u make amendments to anything else.
If not, then try working em out in small parts. Dun play 1 whole chunk/section of em. Sometimes I practice 2 bars by 2 bars if I have to make the whole thing smooth.
