Worst decision I ever made was to agree to sell my house to get the equity to invest in going into a joint venture with my boyfriend, in which we bought two dilapidated properties to renovate. Once completed, we'd live in one and sell the other for a fat profit.
He turned out to be a liar, a cheat, a thief and a con-man who gradually became insulting then abusive, then physically violent, including raping me on several occasions. We ended up in a sort of hostage and terrorist situation, him holding all the power by not completing the work, keeping me cowed and terrified of him and unable to leave, and by then he had ensured that I had no money to pay another builder.
I started with a house worth £66k of which £33k was equity and I had £20k in savings. By the time I got free of him (which took 5 years) I had lost everything, owed £20,000, and had two mortgages, each on a property that looked like a building site and was nowhere near being habitable or saleable.
The story of how I turned all this around is too long to relate, but eventually, after 8 years, by my own efforts, I got myself back to the good, safe and stable financial situation he had originally found me in.
The best decision I ever made was to never get financially involved with anyone again, so nobody can ever take away my home, my security, or my peace of mind. I managed to get rid of the two houses he had persuaded me to buy, and purchase another house. I have lived in it with lodgers for the last 25 years.