Reddit mentions of Francisco De Osuna: Third Spiritual Alphabet (Classics of Western Spirituality) (Classics of Western Spirituality (Paperback))

I would look into what is called "The Prayer of Recollection" You don't really say anything interiorly, but just focus on the presence of the Trinity within your soul. Two good books to understand this type of prayer are:
The Spiritual Doctrine of Bl. Elizabeth of the Trinity

and The Third Spiritual Alphabet The Peers translation is better if you can find it, but it is rare.

I would focus on the Carmelite methods of prayer and Carthusian spirituality. The focus is more on the interior dwelling of the Trinity in silence rather than long laborious repetitious prayers, although those are useful too. What is most important is you focus on mental prayer, the prayer of quiet, the prayer of recollection, and the practice of the presence of God, this is the quickest way to make strides in the spiritual life, that, and frequenting the sacraments, and daily rosary.

The Practice of the Presence of God also an excellent book.