The most common and most likely answer to this crossword clue is the 3 plus 2 plus 2 letter word WOE IS ME
I've seen this crossword clue in Financial Times
W
O
E
I
S
M
E
Buddy explains!
The straight part
We know that "I feel terrible" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case i feel terrible = woe is me.The cryptic/wordplay part
In cryptic crossword there's often an wordplay/cryptic part as well.This part involves a more complex hint, such as an anagram, homophone, hidden word, etc.
Sadly, We don't have an explanation for this specific crossword clue yet.