Your search for success is over. So the next step becomes the search for significance.
First though, as others have said, your "what to do next" is colored by the natural grieving process you need to go through. Accept that your emotions will likely be all over the place for a while. This isn't a time for big decisions, but rather smaller, quieter, times for you to find a new simple routine.
Once your life has returned to (emotional) normal, then you can start the next chapter. Being significant means making a difference, on the people around you, on your environment, and so on.
Like with most things, there's no "quick path". You need to try this, try that, and see what sticks. Local charities are always a good place to start (they can use the help) but they may just be a stepping stone till you find your place. Interacting with youth in some way (school, church, clubs etc) can also be very inspirational (for you and for them.) Kids like to hear stories of success (they're still on that path) and hearing it from someone young - closer to their own age - really sticks.
Most of all, don't panic if you wake up tomorrow with nothing to do. You've got a long road ahead. You don't need to rush. Take some time to just settle down. Maybe go on a trip to somewhere peaceful.
Good luck.
When I lost someone, people at work felt awkward about asking about it, except for one weird guy that did a lot of charity work. Remember being super pissed at him 3 weeks after the funeral, when he said that the first year if the worst. But he was right.
I really regret not going to a psychologist. I didn't know how to explain it to the family doctor to make him send me to one (live in Norway, where taxes are insane, but if you make your case right, you get almost free medical assistance, capped at $300/y). I also didn't know it was worth it even with costs that seemed prohibitive.
It's worth knowing, that psychologists are like programmers - not everyone is good at everything.
Find a couple specializing in supporting grieving people, see at least 3 and decide which one fits you best.
Sending love, hope you get better!
- https://www.youtube.com/watch?v=KIdWjMs-VYk - https://www.youtube.com/watch?v=a4SEVvTs1L0 - https://www.youtube.com/watch?v=SMQ-rJlOpSU
Pay it forward, find a partner and be an amazing parent. It's not just rewarding, it's fun too!
Honestly, money can mitigate a lot of the harder parts of having kids (not getting a break, getting woken up at night when they're little babies, having to hire a babysitter if you want to see a movie at night etc).
Thing with mourning is that now things seem to be over but they will start again eventually. It's all about time. Also I'm sorry you feel alone, I hope you'll find a way to manage that.
Good luck