Your father's girlfriend should have have been able to sell anything of your father's unless she petitioned the court to probate the will and received Letters. If this was the case you should have received notice of that petition. Do you know what assets your father had? Did he own a home? Bank accounts? Did he have a pension? I strongly suggest you wait no longer and either file a petition for probate or a petition to determine entitlement (based on the overall value and assets of your father's estate) and bring all of this to the light of the court.
You need to retain a probate attorney in the area where your father lived to help you. There are many questionable things going on here according to the information you presented and you need legal advice to protect your interests.
The above answer is not to be considered legal advice and should not be relied upon as such. You should consult your attorney for specific legal advice as to your individual situation.
I am sorry to read of your father's passing. It is not unusual for a man to leave his estate to his long term companion. That they were not married does not really matter. To set aside the will, you need to prove either your father lacked capacity (being on paid medication or having a stroke are not enough, absent more facts) or the will was the result of undue influence. The latter is extremely hard to prove, since he lived with this woman for 19 years. In short, you have a large evidentiary burden to overcome. Unless the estate is large, or a doctor treating your father at the time the will was signed is willing to say there was no chance your father was able to sign a will at the time, your successful challenge of the will is unlikely.