Why does having limited time to play throw me off?

[llleisure]

Yesterday I had the chance to play poker but had only about an hour to play and I couldn't do it. I just knew if I sat down and played, I would have the fact of having to leave in an hour hanging over my head and it'd bug me the whole time. I feel like if I have a time constraint - at least a short one - over my head then I can't play properly.

I don't know that this is a terrible problem to have - and at least I'm aware of it - but I am sort of curious why I feel this way. If I sit for a game, I really want to have the option of playing as long (or as short) as I like. I will sometimes only play a short session but knowing that it MUST be a short session throws me off.