First I want to comment that I am making a firm distinction between love (which I have for many people, including those not of my preferred sex, and sometimes non-humans or even plants, and is a sort of sympathy with and recognition of the beauty of the other) and being "in love" (which is seems to be a far more intense, and consuming feeling towards one specific person at a time, I think). I love my friends. Being "in love" is something else. I think it is the case that one must love the person with which he/she is in love (one is necessary but not sufficient for the other), but perhaps not.

Furthermore, being in love (if I can call my experience that) doesn't have anything much to do with being in a romantic relationship with a person.

If you spend a lot of time with someone, you will get more data about them, and the exaggerated view will be fixed

Not necessarily, or we couldn't fall into affective death spirals. There's confirmation bias to contend with.

then it seems unsustainable in long term.

Yes. That is probably the the case in most situations.

From the inside this feels like "realizing that X wasn't your true love, because you have met Y, and now you understand that Y is your true love". Of course, in the next cycle it leads to "realization" that Y also wasn't the "true love". (It is possible that this effect is responsible for many divorces, so I would strongly advise against getting married in this situation.)

Did you see what I wrote here?

I have distinct meta-cognitive thoughts. For instance, I feel like I will love this person forever, because that is entailed in the feeling, but I am also aware that I have no real way of predicting my future mental states and that people who are in love frequently and wrongly predict the immortality of the feeling.

also..

None of this has any extremely obvious effects on my decision making: I wouldn't run off and get married for instance, because of that voice of rational meta-cognition, for example. However, It probably biases me in all sorts of ways that I can't track as readily.