What does Adele mean by "bitterness" in the lyrics below?
When will I see you again?
You left with no goodbye, not a single word was said,
No final kiss to seal any sins,
I had no idea of the state we were in
I know I have a fickle heart and bitterness,
And a wandering eye, and a heaviness in my head
But don't you remember?
Don't you remember?
The reason you loved me before,
Baby, please remember me once more

Question

What does Adele mean by "bitterness" in the lyrics below?
When will I see you again?
You left with no goodbye, not a single word was said,
No final kiss to seal any sins, 
I had no idea of the state we were in
I know I have a fickle heart and bitterness,
And a wandering eye, and a heaviness in my head
But don't you remember?
Don't you remember?
The reason you loved me before,
Baby, please remember me once more

Answer

The feeling she is describing is resent. 
In regards to taste, bitter can mean having a harsh, disagreeably sharp or biting taste or smell. Now when we think of bitter feelings the description is just like that of a bitter food.

Answer

She means a sad feeling of anger towards someone who hurt her feelings.