Sounds as if you are doing ok and learning fast.
I find easiest way to change an O ring is to cut off the old one, then fairy liquid or a drop of oil on the stem and they pop on nice and easy.
Stuart
I have the same thing on a couple of mine, though not bad enough as yours from the sounds of it. It's good practice to leave the whistle & safety valve loose when the engine's not in use but you don't really need to actually remove the whistle so I tend not to.