Let the little things go

that i havent quite grasped how to let the little things in life that eat at me go. things like my friends not being there for me as i am them. people at work not picking up their slack. insignificant stuff that i shouldnt get upset about.
recently my boyfriend told me that if i dont start letting go of the little stuff and moving on he was going to leave me. we are currently apart because we both have things we need to get situated,but i need to know how to do this so we can get back to the relationship we had.