I'm sorry you feel that way, I wish I had anything to add that doesn't sound banal. I think that even caring about doing meaningful work for mankind at all makes you one of the better people on planet earth.
I know you're probably just venting, but it does sound like you're encouraging OP to do it. Also, it is literally not true. We might be gone, but the pain of the people that cared about us is still real. The positive impact that we could have made on others is also not going to happen. I'm going to take my own life (hopefully) soon too, so I'm not going to judge anyone, but we should at least be honest about the impact of our choices.