I kind of get what you mean, I was in a relationship (first proper one) when I was 18 until last year and it was… well always intense both in the good and bad sense, I was totally head over heals and this made me overlook all the problems and stay for way longer than I should have (2 kids probably helped that) then things ended (she cheated) and I was utterly heartbroken after all the things I put up with/took her side when no one else did….
But with hindsight I realise it was never going to last and long term I would be better off without her, I briefly started seeing someone else and tried to go slow/keep things casual but she wanted a lot more and I told her I just wasnt ready and things ended.
But again looking back (always easier) I just didnt feel the same like you said “the highs werent as high” which seems stupid because of the fact of being with someone normal without all the crazy bullshit should be better…