In this sentence, you wouldn't use "about". The sentence would be:
"The thing I regretted most was forgiving her"
"What I most regret is having forgiven her"
"What I most regret is forgiving her".
Also, you should avoid saying "the thing" , and say "What" instead. You could also say "That which"
"That which I regret most is forgiving her".
These would all be more natural sounding ways of phrasing this sentence.