Mia, I bet that you have only a few hours so far, and that your legs are sore the morning after a flight lesson. What you are describing sounds to me like a bit of nervous tension, and the release for you is in tensing up your legs. Dont worry too much about it. It will go away as you get a few more hours and become more relaxed. It is hard to get consistent and accurate pedal inputs when you are tense, but trust me, it will get better very quickly.
If your instuctor doesn't know this, she is unprepared for the task of teaching.
What you are experiencing is normal and it will disappear in a few short hours. I can pretty much guarantee it.
In the mean time, you should carry on with the other lessons, and dont let this hold back your progress in other areas.
You probably wont even realize the first time you have a success here, and after that, you wont know why you were having a problem with holding the centerline in the first place.
I pronounce you "normal".
Cheers Mia
RTR