For anyone confused about this chapter, I'll explain what's going on.
So, after the team succeeds in blowing off eren head, essentially extracting the worm from eren, causing the rumbling to cease. The team thinking there in the clear, finds out the explosion wasn't enough in killing the both of them. As eren transforms and fights back against his former comrades, the parasite (which is the source of all titans, that gave power to ymir in the first place) transforms the people in the walls (ymir's descendants), the Eldians into titans, because they are related to ymir. Battle issues on as eren and the parasite try and reunite together, in order to continue the rumbling. The team now knowing that if they dont kill eren for good this will never stop, they decide to finish him off.
Now this is where Mikasa's point of view comes in, this is essentially showing her in another timeline, one where she and eren ran away, eren didnt act out his plan in what you saw in this current timeline, so the Paradisians came to war with Eldians and are essentially losing the fight against them. Mikasa is questioning her existence being here (talking about this timeline, since she's connected to eren, so eren is essentially allowing for her to see another timeline by the power of the parasite and the sand place where time can transcend in any direction), and eren already know what she's talking about says to her to forget about him, and live a long and happy life. Essentially telling her to kill him, mikasa knowing this, though telling him she wont forget about him, proceeds in doing so, with the help of Levi they successfully kill eren.
This whole chapter was a sad bitter sweat ending to eren and mikasa's relationship and the end to what ymir and eren were ultimately trying to achieve. Eren and ymir's whole goal was to stop the loop from reoccurring and eren succeeded in doing so, which will be explained in the next chapter, taking the point of view of eren. Well that's what I and many others predict to be so. You can also agree to this as in the last panel, you see ymir in the back smiling. This could be hinting at what will essentially be the out come of AOT's ending, it all coming back to the beginning from chapter one.
Plus if your confused about this post and my long ass explanation on the chapter (if you can call it that) then check out some videos on youtube explaining what this chapter means and how it connects to the story as a whole. They are really good in doing these analysis on the chapters.