It may be that your saw is not so sharp. However, it is generally true that
when the angle of the saw and the grain of the wood (on the handle side of a
pull saw) is acute it is slightly more difficult to rip than when it is an
obtuse angle. Change your position until you get it the way you like. You
might consider clamping the wood vertically in a vise with the top of the
wood somewhat below your elbow. Or if that is not achievable you could clamp
it at an angle. Once you get going it shouldn't matter much.

Jack