So, on that last chapter, are we basically seeing three different universes? The first one is the protagonist getting married with the one man who committed suicide in her own timeline, the second is back to the first chapter of her crawling out of the door after failing to commit suicide, and the last one is back to when she was a kid. Or is she just flowing backward again?

I recommend reading Sura's article, "Again, Regrets" which are her final thoughts on it, as it should clear up a lot of things.
Basically we only see two universes. The woman we see in the beginning of the last chapter who married Ho-Min is an alternate universe as a result of the main character's actions to try and change the future. In the main universe, the main character continues to go back in time up until she turns into a kid. The "second" universe that you speak of is actually her having a flashback.