Is it too good to be true?

Have you ever been in a situation, where everything feels so damn right that you start fearing there is something hidden that will soon ruin the happy moment?
I only know that this is somewhat valid.
I don’t know where is the fine line between being paranoid and protecting yourself from disappointment.
I have noticed that the repetitive pattern of recurring troubles has made me too nervous, to the point where daily life sometimes is expectation of the next hard one.
But I am not a good example at all.
How do you personally feel in such situations? Do you believe that the shifting nature of life should always be greeted with a clever dose of suspicion?
But if one does so, how not to make it grow into pessimism?
Indeed, people that do not seek deeper emotions, have it easier because they do not get impressed that much to expect a sudden sharp decrease?
I wish I could elaborate more, but it is simply that so many factors are invisible that I do not feel adequate enough to comment further.