The events of Wonder Woman are essentially a prequel to her introduction in Batman v Superman: Dawn of Justice. However, there's no some question about how well the new movie keeps continuity with the last one. At the end of Dawn of Justice, Diana tells Bruce Wayne that she "walked away from mankind" one hundred years ago. It's clear that she had lost faith in the world of men. Although, even in the trailers of the new movie it's clear that the jaded Diana that has lost faith is not the character we're going to see, so how are we going to get from the one place to the other? Director Patty Jenkins explains part of the issue may be that we have misunderstood what Diana was really saying. According to the director...
Well, I think that I can only tell the origin story of who she is. I think that there is potentially a misunderstanding between what it means to turn her back on mankind... there could be a difference between having people save themselves and knowing that people must save themselves, and even still, being optimistic and loving about them. I think that they were sort of more [synched] up, but we were aware that her snippet [in Batman V Superman] versus the story that unfolded, you know... there is a journey of time between.
Patty Jenkins actually leaves a pair of different possibilities in her response to Screen Rant. First, Jenkins is saying that while Diana may have turned her back on mankind, that doesn't mean that she lost faith in them or wanted to wash her hands of them. Instead, she puts forth the idea that Diana simply realized that she couldn't save them herself and decided that she had to step away in order to let men save themselves.
The director then follows up that idea by adding that there is still a "journey of time" between the end of Wonder Woman and the point at which we see Diana in Batman v Superman. It's possible that whatever events transpire that lead to Wonder Woman turning her back simply haven't happened yet. Jenkins has previously stated that she thinks any Wonder Woman follow-up should take place in the years immediately following the first movie, and if Diana were to end up turning her back in the early 1920's, that's still close enough to 100 years before Dawn of Justice for the math to work out.
While continuity is one of the things that makes cinematic universes interesting, it can also be restricting, forcing movies down predetermined paths rather than allowing filmmakers creative freedom. In the end, it may not be all that important for this single line to be explained.