"I'm still learning to play my instruments (pennywhistle and guitar) so some fingerings are not yet in muscle memory--hence, the ones I know go too fast, and the ones I don't, I stumble through;"

1) You need to practice the bits you have problems with, not the bits you don't.

2) By constraining your self to play at a regular slow pace, you won't race the bits you know, etc. Doing this racing and slowing down will only teach you this bad habit, so don't do it!

3) Play at the slowest pace that will get you thru the whole piece at the one tempo, no matter how slow. Also pick out the bits that are the hassle and just practice them - then start putting it together.

4) Practice the notes you DON'T know more than the notes you do.

Robin