It takes practice. It can be frustrating at times because some days your mind is such a busy place- it just wants to wander from this to that. What you have described is what I do… I envision a picture in my mind as I pray the Hail Mary’s. Usually the faces are from the movie “Jesus of Nazareth” I know some of the mysteries more than others, such as the Joyful and the Sorrowful. I tend to have a harder time with the Luminous (maybe because it only falls on one of the days of the week, while the others are 2).
I use a website, that has broken down each mystery with 10 meditations, one for each Hail mary. I have found it really helpful, not only to grasp the bigger picture, but to meditate on one image for the length of a Hail Mary is much easier than trying to focus on the whole Mystery for 10.
I let the actual prayers melt away as I pray, they are a kind of background noise- what I focus on is the mysteries.
Either way, whether you felt distracted or not, your rosary counts! Just keep trying:)