I think that while this could be about a past relationship, the "bitterness" could refer to hardships unrelated to the relationship. My interp is that it's about people navigating hard times together (possibly leaving something behind? I get that from the history book thing, like a history book is full of the past and things that are gone now?) and looking back at things they left behind before whatever hard times hit them came along. Not necessarily saying that their relationship is over, considering the "i'll be on your side" bit could mean they're still together and with each other through this stuff. My takeaway is that the verses that seem to be telling the story of the people's past and being in love is like them talking about when their lives were better or simpler and then the singer saying that even though things are hard now and it hurts to look back they'll push on and not let go of or forget what they left behind. It's really more of a "i'm gonna stay with you because one day we'll be okay" sort of song more than a "i intend to be back together with you even though we broke up" song to me. And like, the implications of "history book" seem like something bigger than a bumpy relationship. Like history book makes me think of massive, important things and having a huge weight on you that you've carried for a long time ("heavy as a history book can be"). And the "like dogs we'll drive the wolves away" part makes me think the singer and the person they're singing about are in this together, fighting side by side against something bigger than both of them, and then imagining what it will be like when they've finally won and the relief they'll feel. But yeah that's what i get from it, that it's more about people who love each other leaving something behind and fighting something big and the singer promising not to forget the times before things got so hard even though it hurts to remember, and that they'll be on this other person's side in the fight and that one day maybe they'll be happy again.