Answer:
Like, put your tone a bit sad, but guilty at the same time. Some words of regret is "I wish I hadn't done that (sigh)" "(sigh) I feel bad I did that" "I really wish I could take this all back (sigh)"
You see, like, the regret tone is mixed with sadness. Take the "i feel bad I did that". The sigh was sadness, the "I feel bad" was guilty, and the "I did that" was just overall both.