Allow me, if you please, to make a general statement here.
I read the Diary almost every day. I love it. I am active in a ministry committed to this saint’s devotion to Mercy. The other people I know who are committed to this devotion tend to be older, and very well formed in public revelation. In fact, the people I know who are committed to other private devotions also tend to be older, and well formed in the doctrine of the Church.
People who have a solid foundation (public revelation) can build on that foundation, sometimes encouraged or enriched by approved private revelation. I saw this in the early days of the Catholic Charismatic Renewal, as well. The problem I saw later in the Charismatic Renewal seminars is that we were getting younger people who never got doctrinal foundation, even though they went to Catholic schools. We were trying to teach renewal, without any foundation.
Because doctrinal ignorance and confusion is so rampant today, we need to commit all our attention, at least with young people, to foundation building. We may be entering a season when private revelation will need to take a much smaller place than it did before, until we restore the fullness of doctrinal knowledge to the great number of Catholics.