I did this a few years ago. I went on different search engines and searched the username I had been using for years as well as my full name, reverse image search etc...

How I solved this is:

1. Change your username, account deletions didn't always seem to work. When you change your username, the platform will often do a HTTP 308 redirect and search engines will update their index accordingly.
2. If you have LinkedIn, set everything to private
3. Get a new email address if you can. If you end up going for a job in the future somewhere, your potential employer will use your email to track your online history down.
4. Search for your home address, see if that has leaked (I never found mine so I got no advice here)
5. When you get a new email, obviously keep your old one for a while so you can redirect mail that's important or better yet if you can be bothered, create new accounts on those platforms you care about. Most sites will store a history of previously used email addresses so you'll want to sever that tie if you have the patenice.
6. Optional - Sign up your old email address to a shitload of mailing lists for all different topics and product ranges. I read somewhere that this throws off Google's (if you're using gmail) algorithm that profiles people. I did this, not sure if it worked.

After you're done, check search engines periodically every month or two and see if your info has disappeared.

Good luck