I do realise this question was submitted a while ago but I am going to answer it incase people are looking for the answer and come onto this page. It is pretty much impossible (or so i thought) they will just keep fighting, you need a miracle, the romance festival. have your sims be friends again before the romance festival then go together and drink the special cherry blossom tea together (this enhances chance of romance) then click on your ex and select the "joke about getting together" option then from there on click any romantic option you like and it will work without them fighting, the date will go so well it might even end in a proposal. mine did anyway. Thanks hope this helped.
