do i have a case?

If there are no other relevant facts beyond what you've stated in your question, you have a very good case for breach of contract, but you have two problems.  First, since you drafted the contract yourself, you probably didn't know to include what is known as an "acceleration clause", i.e. a clause that provides that if the borrower defaults on any payment, the full amount will immediately become due.  This allows a lender to sue once for the full amount, rather than having to sue over and over as each payment, or a few payments, is missed.  Since you probably also neglected to include a clause that provides that the borrower is liable to pay all costs of collection if there is a default, including attorneys' fees, it can be very expensive, not to mention time consuming and annoying, for you to have to sue the borrower multiple times.  The second problem is that your loan is unsecured, that is it is not like a mortgage, where the loan is secured by real estate and if the borrower can't pay the lender can get his money be having the real estate sold.  You can sue and get a judgment against the borrower, but if he doesn't have the money to pay the debt, you may never be able to get your money, or it may take a long time and a lot of collection efforts, such as garnishing the borrower's wages, to collect.