It's not really clear what the (canon) status of his relationship with Sharon Carter is after the events of Civil War. They kissed one time and he then immediately went into hiding for more than a year. She then gets herself snapped out of existence for five years.
By the time he makes the decision to go back to the past, he may well have not seen her for the best part of seven years and there's no good indication that they were even dating (or even Skype-ing) while he was on the run.
In earlier versions of the script for Infinity War, Steve was actually living with her, but that was removed entirely, with the implication being that their relationship (such as it was) is long since over.
"In previous drafts, a couple of times," Markus told Collider. "Sharon
Carter was a victim of what we came to realize as we were writing it
which is that if you're writing scenes that set up people before the
story has gotten to them such as Steve and Sharon trying to make it
work in an apartment, they're gonna go because the movie does not have
time to catch up 'previously on the life of Steve Rogers.'"
'Avengers: Infinity War' Writers Reveal What Happened to Sharon Carter