It's happening for you not to you.
When things go south many times people ask why? Why is this happening to me? What did I do wrong? Many times there isn't a simple answer but it really doesn't matter why unless there is something you can do with that why that will change the outcome of the situation. What matters most is your attitude about the struggle or difficulty. A small little shift in your perspective can make all the difference.
Next time you have something happen and you begin to ask why is this happening, stop. Tell yourself this is happening for me, not to me. You just lost your biggest customer. Your new perspective says, losing this customer is happening for me not something happening to me. Or in other words, losing this customer will turn out for my good.
Maybe losing your biggest customer (who in reality was too much work for the money) was a good thing because it made way for a better customer to fill the void. A better customer, who is paying you more and the work is easier. When doors close and even when they slam shut and lock it isn't always a bad thing. Many times it takes hindsight to see that what felt like bad at the time, really was a great thing. And we will see opportunities instead of struggles if we can dig deep within and find the courage to say, "this is happening for me".
Practice it today. Each time something goes wrong simply say, "This is happening for me". And really believe it. Watch how it changes how you feel.