I read this book, a paperback, back in 2009 when I was homeless and a meth addict, so my memory of it is sketchy to say the least.
The protagonist is a young woman, given a mission to board a space ship, by a grim looking older woman. She succeeds to defeat alien monsters, and she also has a love interest involved. Then somehow it turns out in future that she herself engineers these creatures and populates their planet.
She ends up living somewhere in this ship or elsewhere off-world, my memory fails me here but I know she does not settle down with her love interest, and witnesses the end of the universe as well as the beginning of a new, identical one.
In the new universe she, in some sort of disguise, gives her younger self of the new universe the mission, so she becomes the miserable older woman. And finally she settles down with her love interest and lives out her days with him.
The only other details I recall is that the book was a quick read, it took only one or two hours, and I thought at the time it was not particularly well written, but I read it because my friend was raving about it.