Since your initial offer to the PhD program came before you'd decided to go with the masters (and since admissions have not requested proof of your masters certificate) it seems that the issue is not whether or not you should be on the program, but rather whether or not it is the right thing to do personally/professionally to let your supervisors or department know.
I think your plan to 'to concentrate on my studies and research for now and, when I feel ready, find an appropriate time to discuss this with them' is perfectly reasonable. You have every right to be on the program with or without a masters, and if I were you I would only reveal this information if you think doing so would be important for receieving extra support from the department, or something akin.
Side point - If you feel concerned that you might struggle with the PhD in the same way as with the masters then it might help to talk to someone about your concerns either way, and mentioning your previous experience would make sense.