Have you checked your fluid? Could be something as simple as low fluid level, of course that would also mean there is a leak or it was previously serviced and not refilled properly. Do you know what fluid is in it? If someone changed the fluid and used something thicker than the spec'd that can make for tougher shifts especially when cold. Also "Baby it's cold outside", if it's abnormally cold where you are this will thicken the fluids and make shifting more difficult, the solution is quite simple, give the car more time to warm up before driving, as the engine idles even in neutral (foot off the pedal) it's spinning the input shaft on the trans and that means it's spinning the gears inside the trans warming the fluid. How's the clutch master and slave? I had a bad master and replaced it with 2 bad masters before getting a good one and actually restoring full travel to the slave. If none of these are the case, then you you might have worn synchro's, used transmissions can be had for less than the cost of a trans rebuild.