To get a fresh start with your Gmail use, it's sometimes necessary to mark a bunch of old emails as read. 

(If you haven't reacted to unread emails in two months, you will never react to them. And if there was something urgent there, someone might have contacted you via other communication channels.)

1) Click the search bar. Write down "is:unread before:YYYY/MM/DD" – type in the year, month and date before which you want to mark your mails as read.

2) When you have searched all the mails, click "Select all" (see picture attached). After this you get a little extra row above the messages saying "All XX conversations on this page are selected."

3) Click the "Select all conversations that match this search".

4) Behind "More", select "Mark as read".

And it's done!