relaxation is the key! of course you WILL get tired from time to time, even if relaxed, because you have to build up your endurance. its like any other muscles...after time they get tired because youre using the fuel in their cells, and when you get low, you get weak and tired. the more you do this though, the more you will build up your muscle's endurance and the longer you can go without tiring. play, play, play!
and pinky bends are pretty hard to do...your pinky is hte weakest finger, and even with help from your other fingers its still tough to get it right, but it can be done...just gotta work on it. thats the ONLY way to get better...practice and play man. work on stuff that is hard for you, and it will make you improve.