As Lee says, having a good comfortable stance is important.
I re-work salvaged lumber, often, and use a hand plane often, rather than r
isk running an old board, with a hidden nail, throught the powered planer.
I've found: Don't think of your planing as ending at the end of the board.
Imagine the board extends a foot beyond what you actually see. Plane to
the end of the imaginary board. Also, focusing pressure at the end of the
board or on the front of the plane, as you assume you're doing, is likely p
art of the problem. As you plane to the end of the imaginary board extensi
on, focus on keeping even pressure on the whole plane, not just the front.
After a while, that imaginary board extension will "reduce itself" to a fe
w inches, when you get the feel of the board being planed flat to the end,
and not curved downward at the end.
There have been times, when I have short boards to plane, I butt 2 or 3 end
-to-end, rather than imagining more of 1 board is there. I like "the feel
" of having that extra length, imagined or real, to work with.
When my blades get dull, I have a tendency to not only make shorter strokes
, but press harder at the end of a stroke. For me, that pressing harder at
the end of a stroke often equates to pressing harder at the front of the p
lane, as well, and I'll get results as you describe.