A partition action would have to be commenced. There will either be a forced sale or one of you could buy out the other's interest out. If an action is commenced, then you and your former boyfriend will each put proof into evidence regarding your initial contributions and what payments were made on the mortgage. Once that proof has been put into evidence, the judge will decide how the proceeds of any sale will be split or what his interest would be in you retained the property.
No way to know for sure what a judge will do, however, assuming you can prove the amounts each party put in and that expenses were split equally the most likely outcome is that in a suit for parrtition there would be a forced sale of the premises. You would each get back your initial investment (if there are enough funds from proceeds of the sale) and then split any equity equally.
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An attorney would have to see all of the documents and any communications between you and the ex to determine what relief you may be entitled to.
You would have an equitable argument to a disproportionate share of the sale proceeds.
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