In the past I have read and practiced a devotion to the 15 Secret Sufferings of Jesus Christ, as revealed to Sister Mary Magdalen of Sancta Clara, but have also heard some arguments against its authenticity. May I practice this devotion?
I can’t recommend it. The life of Jesus Christ, particularly his public ministry and his salvific work on the cross, was a public event. For a seer to claim to have been given information on sufferings of Christ that were until now unknown is problematic. It would be safer to stick with devotions that you can be certain have been approved by the Church, such as the rosary or the Divine Mercy chaplet.