I made a similar mistake. I was idealistic, in more ways than one. My husband was from a modest working class family, I was from a comfortable middle class family. I partly saw this as redistributing the advantages I'd had in life. (If that was patronising, it was meant in good faith).
I put down a deposit for 24% of our house together (because in my 20s I was square, saved and bought one house, then moved and bought a bigger one.) soon after I inherited some money, and got another lump sum when my parents downsized. Net result: in a couple of years I had "bought" 2/3rds is the house. When I earned more than him, I paid a greater share of our bills.
What did my husband do? He used our left over wedding funds to pay off an overdraft - without telling me; took out a second mortgage on the house and then a loan to buy a car which more than a decade later is still in bits; and has repeatedly lied to me about his friendships with women. Among other things. He has very good qualities too. But when I consulted a divorce lawyer I was told that he would get half the house.
The law exists to navigate these emotional problems in the fairest way possible. It won't always feel that way, but the intention should be fair or the law needs to change.
But I am with you heart and soul in feeling how morally wrong your DSp is being. I have to say, the length of time you have been together will make a difference, and could protect your investment in the house too. You must ask a lawyer.
If you don't mind me saying, I'd also thing very, very carefully whether this is a person you want to spend your life with.