I need to sue my ex-boyfriend for a large amount of money.

My ex-boyfriend owes me a large sum of money and he just told me that he is going to focus on something else rather than paying me back (he already started payments, as we have already had a payment plan set up). So, we tried to settle this out of court, but now he is truly treating paying the money he owes me back as an option and I can no longer make accommodations for this irresponsible behavior.