Human usually blinded by love. When they are so madly in love with that person, they no longer see the fault of the person. But when they fall out of love, they will think "how come I will like him/her?" That's normal.
Actually all these are beautiful experience. We take it as a learning point, been thru and done so neXT time we will know what to do and definitely the next relationship we will tend to treasure it. Don't lose heart at those failed relationship. It Will be better The next time round.
Keep the faith and love♡