You didn't say what wood you're using but I'm guessing
pine? Try cherry or even poplar.
Also didn't mention if or how you're making the "stop line"
for the bottom of the pins and tails. A pencil line will show
you where to stop your cut but won't sever the grain
where you'll chisel to later when getting out the waste
in the sockets. A marking gauge with a round, pointed
pin will tear the grain rather than severing it so you've
got the same problem as the pencil line. A double
beveled knife will sever the grain but leave you a
"V" and you have to figure out if "the stop line" is
on the waste side, in the middle of the groove or
on the non-waste side of the groove. A single
bevel knifed line will sever the grain AND give one
vertical face. If your straight edge is on the non
waste side and the flat bevel is against the straight
edge the vertical face of the groove will be your
finish line. OR - you can plonk out some bucks for
the Tite-Mark marking gauge or the Veritas semi-
equivalent. Both have single bevel wheels with
the flat side on the non-waste side.
This is sort of what you want.
----+ +-- Waste Side -->
Here's some on marking
Also sounds like you're trying to chop out the waste straight
down, square to the face of the stock. Chop at an angle
then pair and you shouldn't get the crushing and bending
This'll illustrate things
The bottom of the sockets don't have to be nice
and flat and square to the face since end grain
gluing doesn't buy you anything and just makes
tuning the fit harder.
hope this helps