How to deal with art block.

I am an artist and sometimes I experience art block. It especially happens if I have to finish something before a deadline. I don't take art classes, but sometimes people ask me to paint a picture for them and they are impatient. They keep asking me when I am going to finish the painting and that stresses me and makes it harder for me to get started. I am thinking things like: "I HAVE to get started" and then I simply can't do it. I have no ideas and I don't know how to get started. Last time time I experienced that situation I talked to someone about it she told me to forget the painting that I had to finish and do something that I enjoyed. She told me to work on a different painting for a while because she thought that I would be able to find new inspiration when I did something that I enjoyed and didn't feel the same kind of pressure. It was very hard to follow her advice and I kept on thinking: "I shouldn't be doing this, I ought to focus on the other painting, the buyer is waiting for it" but it got easier after a while. I painted a different painture first and later I worked on the painting that I had to finish.