I want to sell our home but my co-owner doesn't

There needs to be a formal contract, ideally drawn up by a solicitor and both parties need independent legal advice.
Dear Karen,
I bought a house ten years ago with a friend of mine. We own the property as tenants in common in equal shares. About five years ago, I moved out as I was getting married and bought a house with my husband and we have been living in our new house since then. My friend remained in the property and we came to an agreement that she would pay the mortgage as she was living in the house.