What is bitterness?

if you chose someone doesn't deserved to be inlove, you well tasted a bitterness to your life. so be careful to select someone. it's better to be alone than to stay to someone whom not with you at all.

When one is in love it is all sweet and there is no place for bitterness. This is apparent view. But in true love this does exist. The bitterness is to struggle hard to give their love a concrete shape and to try to to be happy couple and maintain the relation as sweet as it was before and contemplation of them are the bitterness I guess.