A couple of earlier posts mentioned "relaxing" and "not over
controlling". That is the key. When I first learned aerotow many years
ago, I too found myself oscillating behind the tow plane and needing my
instructor to rescue me before I broke the tow rope, was cut loose by
the tow plane, etc.

Then the instructor told me, "Next time the tow plane gets a bit high
[or low or right or left] don't do anything for a second or two and
you'll be amazed at how you come back into position." He was right. And
it makes theoretical sense as well. Without getting into control
theory, suffice it to say that while some feedback (correcting for your
position) is needed to maintain stability, too much feedback will
guarantee oscillations. The trick is getting the right amount and
newbies tend to overcontrol. Oh yeah, my instructor also told me to
stop holding the stick with a "death grip." Part of the same syndrome.
Try to relax more.

One other point that might make you feel better (tho I suspect all the
now proficient glider pilots saying "that was me at first too" helps
even more): Learning to fly aerotow is learning to fly formation.
You're lucky that you already have PP-SEL. A totally new glider pilot
is trying to learn to fly, and fly in formation at the same time!

Lastly, welcome to the club. I'm sure that in not much time you'll look
back on this problem (and any others, like getting dizzy in thermals)
and smile at how far you've come so fast.

Martin