This is going to spoil a lot, but this what efbloggers have to do. I found myself saving URLs for later, and it's a good thing most people mark the episode in the summary. So, Moe-moe writes:

With the sudden breakdown of Chihiro last episode, I was certainly surprised at her speedy recovery throughout this episode. She seemed to go from the hysterical girl 4 years ago and throwing her diary at Renji, to somehow falling in love with him again in what seems to be the span of a day. Is that what Chihiro does every single day when she wakes up? I find it rather hard to believe how she can somehow remain the same type of character that she is if she went from being confused and frustrated to being the calm and cheerful girl that we have seen in the earlier episodes. I can only imagine how much she actually has to read in her diary just to remember all the things from a certain point of time to now.

This is not how I saw it. I do not claim that my understanding is any better, in fact it's probably worse, but here goes.

Chihiro took about a week or two to come to her senses, with every meeting counting a day or more. However, it was very clear that due to random circumstances of the moment she awakens, her personality differs every time somewhat (this is, in a way, the scenario of Mahoraba). It seems to me, however, that she then molds it subsequently by letting herself forget things. A person can retain up to seven things in short-term memory, so Chihiro has complete control over them.

This process may be related to one other detail: I'm quite certain that she spins things in the diary. It came up first during all the pre-reset kissing and confession hubbub, when she obviousy has read what did not really happen (ironically, I do not remember the detail itself, because I failed to note it down). It also happened after the reset and more explicitly. At that time she completely handwaved it on the spot. Honestly, I started to worry that Himura tampers with the diary.

Regardless if she does this on purpose, she exhibits a certain flexibility in what she remembers. Renjii is very lucky she does not choose to remember nasty things.