This is one of my heavy sets from today's leg workout. This is a tough one. I'm right there with you! I'm working on this one myself and am trying to go heavier and really challenge myself.
ALTERNATING REVERSE LUNGES = 3 sets of 10reps at 135lbs
SPLIT LUNGES = at least 10 (On the final 3 sets go for 20 and hit complete fatigue... be the superhero you know you are!!!)
Add more weight after being at the same weight for a couple of weeks. NEVER stay at one weight forever -- your results won't change. Continually challenge yourself to improve.