This wikipedia section describes the idea better than I can: https://en.wikipedia.org/wiki/The_Unbearable_Lightness_of_Be...
My personal takeaway is that, horrifying or not, you cannot escape life, whether by hiding from the world or suicide, you must embrace it, because you will always live your life and there is nothing else. Further, questions of free will aside, you should live the life you would want to live again and again. Let the weight be your guide.
I think the alluring plausibility of eternal return helps sell the idea, but it's best to simply use the idea as a way to think about your attitude toward life, and not be too concerned whether it is a physical reality or not. I subscribe to pragmatism above all else.
If there was a thing as an eternal self I would find that idea terrifying rather than motivating, personally. But I tend to lean towards the concept of non-self. Even if the cosmology of the universe means my life will be repeated infinitely, there's no continuous "me" that experiences it. Just like how running the exact same instance of Conway's Game of Life a million times doesn't mean there's some essence that persists between each instantiation.
So for me, that idea doesn't do much for me either way (but I may also misunderstand it). Maybe sometime in the vastness of infinity there will be another instantiation of "me", but I don't think there's any connection between that instance and the current one (besides the curious property of an identical life).