You are asking about this now, so yes getting some counselling is still relevant because it's still affecting you now.
It's such a big deal and your parents were dealing with their own not insubstantial grief and probably struggling to cope. Them saying you didn't want help them is probably more to do with the fact they were so wrapped up in their own issues at the time.
At 18 you don't want help. It's a fairly standard response. You think you are a superhero but really your not but you lack the capacity to recognise it at the time.
You are just finding your independence and freedom so that's what you do. And everyone else just sees you going out, partying and making the most of life. Often because they don't want to see that your crashing and burning because if they acknowledge it they have a responsibility to try and intervene.
You were doing what a lot of 18 year olds without that burden do. And tbh theres no guarantee that if your brother hadn't died, that you wouldnt have done the same thing anyway. You might have just gone off the rails anyway. You don't know. So untie this to your brother's death.
You certainly wouldn't have had the capacity and life experience to be there for your parents so don't blame yourself for that one. Would you expect your daughter to be there for you now if you had a significant life crisis or would you keep it to yourself?
There's significant things me and my parents have never talked about. We never will. They just aren't those kind of people anyway. It's still too raw for them anyway. That's why you've never spoken about it - it's still too raw for both parties.
20 years sounds like a long time. It's not when it's something so huge.
Forgive yourself. It's done. It happened. It's a part of your life that's gone. Focus on where you are now.
Without the experience you HAVE had, you wouldn't be where you are and perhaps as able to be the Mum to your daughter that you are. Your wisdom and your experience though hard can also be perverse gifts that make you all the better in terms of where you are in the long run.
Look forward. Not back. x