If my ex and I were in a same sex relationship and we have split up, can she force me to sell the house?

It's unclear from your question whether you and your ex both own the home and have equal equity in it; however, it appears as though you and your ex are co-owners of the property. In Texas, this is known as being tenants in common. This status means that you and your ex each own an undivided one half interest in the house. For more information about this type of ownership, please visit this website: http://recenter.tamu.edu/pdf/1815.pdf