In oct 2007 I loaned money to my now ex-boyfriend to buy a vehicle for a business that he wanted to start. 31.000 he had given me a detailed letter about the loan and the promise to repay in full by October 2011. He refuses to pay me back. How can I go about it on my own without an attorney ?

You may without an attorney file in small claims court but the amount you may sue for is limited depending on where you are at. If you loaned him 31,000 dollars this is far above small claims court ( that I am aware of) and you will need to really hire an attorney to file a law suit in court.

... this is far above small claims court ( that I am aware of) and you will need to really hire an attorney to file a law suit in court.

Well, technically you can do it yourself, but for that kind of money you may very well find an attorney who will represent you and, if you do, your chances of collecting are probably greater.