The XP Cap is cryptic, but I am positive that it goes as follows:
The message has three parts: the first part says how much you progrided and how, the second part where you are at your level and the third part if you have some bonus or penalty or not.
The first paragraph, telling how much XP you earned since your last rest, will tell you if close to nothing happened, if you could have performed more, if you feel satisfied, quite satisfied or proud of what you have done. Also, if you dedicated to RP instead of combat, it may say that close to nothing happened, but it was fairly eventful or fairly eventful and insightful.
The second paragraph will have two parts.
The first part of the second paragraph will tell how far you are in your level: if you recently attained your new powers, is beginning to feel more comfortable, if you thrive in your learnings or if what was once a challenge, now seems trivial. The developers do not disclose how far each one is, but I’m fairly sure it is equally divided in blocks of 25%.
This part, however, changes if you did not earn enough XP, becoming “you have yet to master your newest abilities” in the first half of your level or that you are “far along” in the last half of your level.
The second part of the paragraph will tell if you have any XP penalty.
Basically the messages are “you have a strong desire for new adventures” for bonus, “whatever the mist throws at you” for normal XP, “you should slow down” for a first level of penalty, “blind drive” for the second cap of XP and "absolute limit" which even nullifies the XP earned in combat (the dreaded red cap).
This message informing of the cap when you are in the last quarter o your level changes a little, saying that you are mentally exhausted and may benefit from slowing down a little (the soft cap) or that if you keep pressing on, you may not be able to grasp that last bit (blind drive).
Also, I can tell you, by experience, that it is completely normal to remain entering and leaving cap in an eight hour period. I had a PC that I kept touching Slow Down and leaving for a week or so. It has many variables, sure, but in my experience, it is possible to lose cap in eight hours as long as you do not pushes on the cap. And yes, many times only on kill already puts you in cap.