I think since its the newbie first time with any sort of pto collars.
I suggest that you first change your pto selector to rear only and not mid pto. This will allow you to rotate the mid pto freely and get a better look at your collar. Now I suggest that you grab the upper portion of pto shaft and slide it back onto the pto shaft and hold a little bit. this will release a little "locking tension" of the collar. next grab the collar with your free hand and pull it towards the deck. (with pto shaft now free to spin you can wiggle things around to make collar slide back) .
I bet once you get it off you will see how easy it really is. I suspect that the pto collar is dry of grease making it difficult for the locking ball bearing to move down to unlock it. Once its off - look inside the pto shaft and you will see a bearing there.