Legal Question in Real Estate Law in California

You need to talk to a lawyer in person. She is certainly wrong that she gets sole ownership of jointly owned property just because you moved out. What your exact rights and options are, and the costs of various options for enforcing your rights, however, is all going to depend on the details of the property purchase, how ownership was funded while you were together and what has happened since you split. That can only be evaluated in a personal consultation with an attorney.

Well, to start out, your question contains an apparent contradiction: You say "my house" and "I bought", then add "with my girlfriend". The key questions will be: (1) Which of you provided the down-payment? (2) Which of you provided the cash for mortgage payments (if any)? and (3) Of lesser importance, whose names are on the recorded title and the mortgage? By the way, it makes almost no difference who lived or lives in the house, nor for how long. The real question is which of you (perhaps both) made the investment to acquire the (equity interest(s) in) the house?

If you put up 100% of the down payment, and made 100% of the payments on any note and deed of trust used to finance the property, you are almost certainly entitled to full ownership and possession. If, of the other hand, the former girlfriend put up 100%, she is the owner. Finally, if you split the equity part of the purchase price, say 72% - 28%, you are 72 - 28 co-owners.

Now, as to legal remedies: If you own 100% under the above formula and are also holder of full record title, your former girlfriend has no ownership rights and presumably could be removed from possession by an action for trespass or ejectment, which is relatively simple. If you own 100% but she is "on title" down at the Hall of Records, you'll need to bring a "quiet title" suit to get title corrected to show you as the sole owner. Finally, if the equity ownership is split between you, e.g. 72%-28%, the proper legal action is probably going to be a "partition" lawsuit, which asks the court to order the property sold and the net proceeds divided between the (former) co-owners in a proportion the court finds fair, based primarily upon the parties' initial investments, but with certain adjustments for items like payment of interest, property taxes, required maintenance, certain improvements, etc.

In any case, you are probably going to be headed to court, but the nature of your remedy will depend upon several factors not made clear in your question, and primarily by who made the investment in the house.