You have read over tens of books and did highlight lots of content. But Kobo Glo / Clara or any other Kobo ebook readers don’t allow you to export highlights in bulk. You would have to go over each individual book to export them.

This solution should work for any Kobo ebook devices.

Download DB Browser for SQLite (available for Windows, Mac, Linux)
Connect Kobo ebook reader to your computer via USB and confirm USB connection on Kobo device
Launch DB Browser for SQLite
Use Open Database in DB Browser and find KoboReader.sqlite database under directory/folder KOBOeReader/.kobo/ (where folder path can differ based on your device and operating system, but look for the hidden .kobo folder)
Find Bookmarks table and use context menu (right click) to Browse table
Scroll to right and find Text column
Select all rows and use context menu (right click) to do Copy (CTRL+C)
Paste the copied context (CTRL+V) into text editor of your choice and Save
Enjoy

Advanced solution

For advanced solution try running this SQL query in Execute SQL tab:

SELECT Title,BookTitle,Textfrom Bookmark LEFT JOIN Content ON bookmark.ContentId=content.ContentId;

This will provide three columns with a chapter title, book title and the highlighted content that you can copy/paste in a sheet or table.

Automated alternative

Download this Python script that exports highlights. It exports both annotations and highlights from a Kobo SQLite file and comes with a web UI you can use in your browser locally.