In control system jargon we would say that you have more than unity
gain at 180 degrees of phase shift. What that means in English is that
by the time you recognize you need to make a correction your corrective
action is too much. So you make another corrective action to fix the
problem the first action, using even more control input. This results
in an even larger deviation in the opposite direction. And so it
goes...

Add to this that you have three control surfaces that you are
manipulating (you may feel like they are manipulating you!).

The phase shift part means you are late in recognizing you need to make
an adjustment.

Try making more timely (reduce the phase shift), but smaller
adjustments (reduce the gain). Remember, once you get a large mass in
motion (such as a turn to bring the glider back to center line) it
can't stop immediately, so you have to anticipate stopping it with
control action in the opposite direction.

Start out by towing in the morning before thermal activity starts - it
is much easier to maintain position in calm air.

Don't worry: with practice you will get comfortable with towing.

Good luck!

Tom Seim