Not to ask the obvious, but you have double checked your clutch is disengaging fully? 3rd, 4th, and 5th are hit easily with minimal expansion of the clutch plates compared to 1st and 2nd, so it may be a place to start looking. Then check the shift shaft for any bending by using a straight edge along the side of the shift lever and pushing it up and down to see if it stays square. Barring that, the next place to look unfortunately would be for wear on the shift cam and associated forks and/or the mating surfaces of first and second. The comment about it being noticeable first on left tilts would lead me to believe you have wear on the internals but you won't know what until you tear it down. Just standing still I'd see if shifting is different with a hard tilt to the right compared to left while rocking it back and forth. Shouldn't really matter one side to the other unless something is worn down. I'd also do an oil change to inspect the oil for more than normal wear.