I have kept Shabbat for two and a half years. I stopped doing this almost half a year ago. I know that Gentile who does this deserves death according to Talmud. Any sources that can give me a little encouragement?
"A Gentile who made his rest day like Shabbat even during one of the days of the week deserves a death sentence... although the death sentence is not applied, he will be punished by stripes or some other type of punishment" (Maimonides, Laws of kings X). There is no greater "cancellation" of Shabbat than resting like Shabbat on Tuesday, and it is also forbidden, so obviously just doing one Melacha on Shabbt does not cancel the prohibition of a Gentile observing Shabbat - especially since a Jew doing one Melacha is punished for that one deed and is not said he cancelled the whole Shabbat (which would be the case if he had transgressed all forty forbidden Melachot)! Some rabbis go into great pains in order to allow Gentiles to observe Shabbat - Rabbi Haim Richman says "don't call it Shabbat but a Celebration of Creation" (!), another one says "it is not forbidden if it is done under our supervision" (!!). The ONLY case when a Gentile is to observe Shabbat is the "Ger" mentioned in Exodus 20:10. It cannot be the Ger Tzedek who is a Jew, and it cannot be the Ger Toshav who is a non-Jew and does work for himself. So, as Rambam explains in his Laws of Shabbat, the verse is taling about a resident stranger employed by a Jew or hired seasonally. He may work for himself, but we are commanded to let him rest like we do on Shabbat in our home.
Our Patriarchs "observed the whole Torah" but NOT the specific commandments we received at Sinai - meaning they kept the SPIRIT of the Law in all their doings.