The problem with this is that the deep trauma she endured which caused her to behave as a borderline psychopath is being dumped in high school. I am not saying that such an experience is a pleasant one, or that it leaves no emotional scars, but I am saying that the levels to which she is traumatised are way, waaay too high for such an affair.
Just to illustrate my point, let us return to one of the earliest WHAT THE FUCK moments, the earrings. Maki gifts Liz some nice earrings. Liz suffers a mental breakdown, because it is a deep trauma (let me reiterate, all of this stems from being dumped by her high school girlfriend). Maki proceeds to apologise (for doing nothing wrong, she had no way of knowing that particular shape of earrings had any sort of traumatic meaning for Liz). Liz responds to the apology by slapping her, because that is what a mentally stable person does under such circumstances. And Maki's first thought afterwards is whether or not Liz is alright, because of course a chronic doormat like her would think that.
Liz is being a "victim" of a failed high school romance, for which she proceeds to vent in massively disproportionate ways on people who did her nothing wrong and showed her only kindness and love. She is not exactly the shining example of a flawed but sympathetic character that makes you root for her happiness.
I don't think there's any amount of trauma that makes Liz's behavior towards Maki justified. I find it just as compelling and believable that a mundane event had such a strong effect on Liz.
I don't think Maki's apology is framed as a "correct" response, either. Her character arc is learning to value herself. She's not at fault for what's happened to her, but she needs to learn to assert her worth and boundaries.
Maki doesn't have any obligation to forgive Liz or even hear her out. A good ending to the story could be Maki choosing to cut Liz out of her life completely.
I just don't think that's the only satisfying ending to their story. Maki forgiving Liz on the grounds that Liz needs to do better and fix her shit wouldn't compromise either character arc in my opinion.
Now, whether or not that could be executed in a satisfying way is going to at least partially be down to taste. I don't think it's too late to show them doing the work and earning a happy ending together, but that would require breaking out of the soap opera vibe we've got going right now.
So yeah, if you wouldn't forgive Liz in that situation, that's that's 100% justified. But I don't think it's a weaker choice for Maki to forgive her.