i had the same problem as you when i learn my driving last
year,
and from wat i see u already know where ur weakness,
i share my experience with u, i know i can't do well when traffic
is heavy also
so before i book my test i seek advice from my instructor,
which time slot is better and his answer is dun book the 1st and
last slot,
so i choose the 1.45pm warm up and 2.45pm test slot,
and on my test day the road is clear and i can slowly do the
changing of lane
to do U turn and move to extreme left and back to extreme right
again to make a right turn with no problem at all. And i had passed

I admit i m not a steady and confident driver so have to play with
the time slot lor, heehee..
It is also good for u to book 1 or 2 practice on the same day (eg.
test date on tuesday, book practice on tuesday) and same time slot
as for u to familarize the traffic condition on that particular
time.
BTW which slot u booked?