I have an online keyholder and a girlfriend who does not know about her

I have had an online keyholder for about a year. Never met, never will, different continent, no romance in it as far as either of us is concerned. It is check-ins and rules and that is genuinely all.

In March I started seeing someone locally and it has got serious faster than I expected. She does not know about the arrangement. It was easy not to mention in week two and it is very hard to mention in month four.

I am not asking whether I should tell her. I know. I am asking how, and whether anyone here has done this without it going badly.