Getting a tump set up correctly on a canoe takes some time, but is worth it. I trip with tumps exclusively. My main canoe is a 15 foot prospector in wood canvas and is around 70 pounds with paddles, fish poles, dog matt and life jacket tied in. I tump that with a 30-35 pound pack on my shoulders first load, and take my second load at about 60-65 pounds, large canvas bag trumped.
sounds like your tump may be a little short, not deep enough ... should not or barely hit you shoulders, really thwart should be much lower, just below or on your traps, when canoe is slid back a bit with tump on you.
pm me if you have questions, every canoe I have had its own tump. It is a nearly lost art in my opinion,and if not done correctly can be painful and dangerous.
Bob