Don't push yourself, is my best advice. Everyone learns at different speeds and will plateau at different times as well. Just because the other person is getting better each lesson and you feel a bit behind doesn't mean you need to rush yourself to catch up.
My first jumping lesson I went from a tiny cross-rail to 3'3"... scariest thing in my life and if the horse I was riding was not a solid citzen, I would have been ruined (confidence wise) and probably still be riding dressage, luckily my mom had a good head on her shoulders, and so did my previous trainer, and we found a new jump trainer, and I've been with her ever since.
You'll always have your highs and lows, just take your time.
Gordon Wright once told a student to take up swimming because he would never be a good rider, that rider was George Morris.