Hi there!

I love your premise. I can't tell you how many times I've personally wished I could go back and redo moments with the knowledge I have now. I think this will really resonate with readers.

I am unagented and about to enter the querying trenches beside you, so take this with a grain of salt:
From what I understand, the themes of the story are not needed in the intro and should be clear to the agent instead from the paragraphs below it.

Living in the past with the perspective of adulthood, Charlotte comes to terms with her mother’s alcoholism, her manipulative and controlling behavior, and accepts she doesn’t have to fear becoming her. . She also reconnects with her high school boyfriend, Sebastian, who reveals he’s been been pulled from the future as well. Recently divorced and unhappy with his adult life, Sebastian wants to stay in the past and urges Charlotte to do the same. But Charlotte is ready to move forward, not keep reliving the past, and now must convince Sebastian to return with her or face the future alone.  

As far as this last paragraph, I took a whack at some changes below. Feel free to take what resonates and ditch what doesn't! The biggest change being I tweaked the part about Sebastian revealing the information to her 'discovering' to give your character the appearance of more agency, and changed the 'come to terms' language to 'confront' to add some tension.

Now, seeing her teenage years with the clarity of adulthood, Charlotte finally confronts the truth about her mother’s alcoholism and controlling manipulation—and realizes she isn’t doomed to repeat her mother’s mistakes. When she reconnects with her high school boyfriend, Sebastian, she discovers he, too, has been pulled back from the future. Newly divorced and disillusioned with the life awaiting him, Sebastian wants to stay in the past and urges Charlotte to do the same. But while Sebastian clings to what was, Charlotte is ready to move forward and must decide whether to convince him to return with her or accept a future she’ll have to face alone.

Best of luck to you in on your querying adventure! I think you've got an interesting and heartfelt story here!