You can’t concentrate on both things at once. If I’m concentrating on the words of a prayer, I cannot simultaneously think of a mystery, and if I am deep into a mystery meditation, I am saying the prayers as background while I think about the Nativity or whatever, which means I am not concentrating on words of the prayers.
I recommend that you perhaps try just reading or thinking about the mystery for a couple minutes at the beginning of the decade and then say the prayers. Or use a “scriptural Rosary” meditation where in between each Hail Mary you read a line of scripture or a line of meditation. Lots of them online for free if you want to try that.