Is the noun “regret” countable or uncountable? Why is it "regrets" here?

What is y’all?

"Y'all" is slang that can be used when speaking to one person or multiple people. Ex: Let me sing it for y'all!

Is the noun “regret” countable or uncountable? Why is it "regrets" here?

"Regret" is a countable noun, so "regrets" is correct. Ex: I have a lot of regrets about how I handled that situation.

What does "fast lane" mean here?

"Fast lane" has a couple of meanings. The first meaning is a lane of a highway for use by traffic that is moving faster than the rest. Ex: Dad merged into the fast lane to pass someone. Ex: I hate driving in the fast lane. I don't like driving so fast. Another definition of "fast lane" is a situation in which life is hectic or highly pressured. This is the definition that is used in this video. Ex: He is living life in the fast lane. Ex: She has always lived her life in the fast lane.

What does "wintergreen" mean?

"Wintergreen" is the flavor of breath mints she is talking about. "Wintergreens" are a family of plants which have a minty flavor, so it is a common flavor used for breath mints and gum.

Can I say "heard about" here instead of "heard of?" Is there a difference?

There is a difference, yes! So I wouldn't use "about" instead of "of" here. To "hear about" something means to be informed about the details of something, whereas to "hear of" something is to know that it exists. You can hear "of" something and know nothing "about" it. Ex: A: Have you heard of the famous bakery in the city? B: I've heard of it, but I don't know anything about it. Ex: A: Have you heard about BTS? B: No, what happened? A: They're taking a break as a group.