can i sell my house because of a broken contract agreement after they had paid me some money, but not all of it?

I sold my trailer in Adams County, CO to a woman named Dawn Solomon. The agreed price for the house is $20,000. Dawn only paid me $ 15,000. I have in writing, on the the sale agreement, that the rest of the money would be due in 30 days of the signing of this agreement. Well, it is March 2015 and she still hasn't paid me the money that she owes me. I sent her letters ( some certified) and called, and no response and no money. Dawn has a lawyer, she says it is illegal for me to sell the trailer to someone else because we have this agreement. It's been almost 2years that she has owned me this money. I think she broke this contract and I have the right to sell it to someone. I still retain the title for the house. I told her I would send her the title when she paid me the rest of the money. I was informed that in the meantime, she sold the house to Dawn. This lady thought she was buying my house from Dawn. She gave Dawn $10,0000 to pay off what she owed me but Dawn didnt giv