Patience

When you just can't fall asleep, it's no use trying to force yourself to sleep. The more you try to force yourself, the more sleep will elude you. The same applies to many other things: it is not good to force yourself too much, because the more you try to force yourself, the stronger the opposing forces will become. 

At times this applies even to prayer and devotion. You should certainly make every possible effort to sanctify yourself and attain true devotion. When something can be done today, don't leave it for tomorrow - because the world never stops for a moment. Man's world consists only of the present. Whatever you can do to serve God, do immediately and determinedly without delay. Who knows what obstacles you may face from the outside and from within if you leave it until later? 
But at times you may see that, despite all your efforts and determination, you simply cannot achieve what you want. Sometimes you must simply wait. Don't be discouraged because you are not achieving what you want. Don't let this push you off course. You must wait a little until the time is ripe. 
The most important thing of all is to look to God at all times with longing and yearning, even when things are not going as well as you would like in your prayers and devotions. Never despair, no matter what. And as soon as God gives you the opportunity to do something holy, do it at once. 

Rebbe Nachman, Chayey Moharan #431