With your legs, not your back

I have a hard time with bending my legs and not my back when doing the leg routine.  Not all the time, but sometimes.  Its just that I have little legs, and doing all of the reps takes a lot out of them.  I'm confident that soon I'll be able to really get low in the squats and lunges.  During some of the exercises my legs burn so bad I feel like falling over!  That's when I come up a little bit, or back off until I can go back down.  Every week I do a couple more reps here and there; which helps with the motivation to keep doing it.  Ab ripper was tough as hell, just like always, but still did a couple more reps with em.  Also finished all of the mason twists, including the bonus :)  That makes me happy.  I need to really push it during the pulse-ups.  If I can get those down, I'd be doing alright lol.  Next week is recovery / ab focus.  Kinda looking forward to that break from the crazy lifting routines, but don't want to get lazy on an easier week.