I know the rule. That I should be professional and keep my personal life for me.
That is indeed the rule, unless it affects your performance
Anything that affects your performance is a concern for management. The function of management is to expedite matters to ensure successful execution of duties and removing obstacles to that end.
When I was a project manager, I would say to my people. "Raise any concerns and come to me with any problems as soon as you discover them. Remember, before the deadline it's a "concern" after the deadline, it's an excuse."
The same thing applies to anything that affects your performance, and yes, this includes your personal life. Things happen off the job. People get sick, deal with cancer, the death of a loved one, finances, et cetera and it effects our performance.
When my uncle passed away, I went into work the next day and misspelled a coworker's name in a message. Then I sent another message and misspelled the word misspelled. Then I let my manager know what was going on and went home.
The time to approach your manager is now, and you can phrase it like this:
Hey, boss. I'm going through some problems in my private life right now, and I'm concerned that it might start to affect my performance. Could you please let me know if it does? I won't burdon you with the details. I just want to keep on top of things. Thanks.
Your boss may or may not inquire further, but you've made him aware of an issue. This sets the tone. He will likely let you know if he's already noticed something.
Taking this approach is very professional. Remember, being professional and being human are not mutually exclusive. Letting your boss know that you're having a problem is not unprofessional. It's being responsible, and being responsible IS being professional.