I am writing a fantasy book and I've recently discovered that another story which already exists has a back-story that somewhat resembles the back-story of my world and I don't know what to do. Most of the back-story can't be rewritten because too much of my story depends on it, without it I can't write my story. Of course I also don't want it to seem like I stole it (which is impossible considering I made up my story before the other one was ever seen by the public and I only recently actually saw the other story) and I don't want to tell the same story twice. However I also don't want to abandon this story considering that I've been working on it for quite a long time.
The story I'm talking about is a video game series called Assassin's Creed. If you don't want to be spoiled on it, don't read what comes next.
I've only played the first game in the series myself but I've recently discovered that in this series' back-story there was a group called "the first civilization" which was a civilization of humans that evolved on earth that became very technologically advanced and then created and enslaved our "species" of human until their slaves rebelled and won due to a numerical advantage. They also interbred with some humans to form "hybrids." They were then mostly destroyed by a solar flare until thousands of years later one of them attempts to come back. This one needs to be freed to stop cataclysm from occuring (a similar solar flare event as the one that destroyed most of them).
In my story there's also another group of humans that evolved on this planet that was more advanced (magically advanced in this case) than we are (although they didn't create us) which enslaved some of us. The humans also rebelled and won in part due to a numerical advantage, they also interbred with the humans and they were also devastated by a cataclysmic event that threatens to cause havoc in the story's present and may require their return.
I should note that these are the only similarities between the two stories that I could find. For example in Assassin's Creed the past is hidden to humans, while in my story it's known by everyone and this history has a significant impact on how human civilizations develop. Also my hybrids comprise about 1/5.000 of all people and are recognised as being hybrids and descendants of hybrids by others, while in Assassin's Creed the hybrids are rare and the fact that they're hybrids isn't known to anyone. Also the hybrids in Assassin's Creed have a few low-key powers, while the hybrids in my story have a large array of impressive powers. Of course my story also doesn't focus on assassin's or anything like that.
So do you guys think it sounds too similar to work? Does anyone have any idea what I could do? Perhaps some way to make it less similar? A workaround? A way to emphasize the differences vs. the similarities? Is it not a big deal and am I overreacting?