Are you lubricating the shaft before you put the seal on? I prefer a heavy oil to grease for that, but any lube is better than none.
For a seal I had to slide over a long shaft on my Coot, I wrapped light card stock around the end of the shaft, slid the seal on to that, then slid the seal and card almost up to the seal's final position, then slid the seal of the card stock and onto the shaft. This protected the seal from getting damaged while sliding it along the shaft. A very thin sheet of plastic might work as well (cut from some junk that might otherwise go in the recycle bin). THere was some trick I did to get a grip on the card to slide it, but it's been a long time, and I can't remember what I did right now.