All Too Well

You'd been with the avengers for a long time, joining the team the same time as Bucky. You were born with special abilities, eventually dubbed 'Siren' by the team. Soon after, you and Steve fell in love. Until Thanos snapped away half of the world's population.
After the war, just as you'd begun to think the world could right itself, that you and Steve could be happy again, your world broke apart even further than you could have imagined.
Steve left you, left everyone, to return to the 1940's with Peggy.
What are you to do when he comes back to your time and finds you having left the avengers and now raising your child alone?

** Vaguely inspired by All Too Well from Taylor Swift**