Valve O ring Help

Hello everyone,
I have noticed that my valve O ring is bad. I have been working on replacing it for an hour. I just cant mkle it fit in .
Was hoping someone has a trick on how to place the o ring in the brass valve. One other thing does anyone have an exact size on that o ring?
I’m at the point of just ordering a new one it’s that hard to fit in. Any help you can offer would be appericated. Thanks Kip.