It would  be hard to choose a worst feeling as there are many that cause pain & hardship.  But regret has to be up there at the top.  Regret leaves you feeling helpless.  It implies that something bad has happened to you or a loved one and there's something that you could have done but did not do. Acting can lead to a bad outcome, but at least you acted.  Regret often follows inaction.

To be betrayed by one you love is an indescribable hurt.

The bad feeling when someone wrongs you.

I think the worst feeling one can have is lack of compassion (for yourself and others).

the worst feeling is when your best friends or the person who is special for you is not with you when you need him/her.