I believe that unless you are chemically, neurologically or biologically imbalanced, there's no excuse to be bitter or unhappy. I won't discuss happiness here, but even if the advice of some bitter person is good, I would say that an advice of a person who I lookup to will always be "better".

Of course, things should be placed in context. An advice of a bitter math teacher on mathematics will always be better than an advice from a happy ignorant person.

On a personal note, don't be bitter. You can accomplish the same without the bitterness, it's just weighing you down.