Dear Lucy:

Internal obsession is essentially what is described in that footnote, "influencing the imagination in particular, seems to force upon a poor soul something of his own sentiments..."

While the exterior obsession is more dramatic and spectacular, it is far less dangerous than internal obsession, which affects the interior faculties. I will quote from Fr. Jordan Aumann's book, Spiritual Theology, but with the warning this is for informational purposes only. One should not try to diagnosis anyone, including oneself. Diagnosis needs to be left to those who are trained and experienced to do so:

Obsession can affect the interior faculties, especially the imagination, or the external senses in various manners and degrees. The attack on the imagination differs from ordinary temptation only by reason of its violence and duration. Although it is difficult to determine exactly where simple temptation ends and true obsession begins, we can say that when the disturbance of the soul is so profound and the tendency to evil is so violent that the only possible explanation lies in some external force (even when there is nothing evident externally), it is certainly a case of diabolical obsession. It can take many different forms.

Sometimes it s manifested as a fixed idea that absorbs all the energies of the soul; at other time the images and representations are so vivid that the subject feels that he or she is dealing with concrete reality. Again, it may refer to one's duties and obligations, toward which one feels an almost insuperable repugnance, or it may be manifested by a vehement desire for something one is obliged to avoid.

This seizure has repercussions in the emotional life because of the intimate relation between the emotions and the cognitive faculties. The sou, even in spite of itself, finds itself filled with obsessive images that arouse doubt, resentment, anger, antipathy, hatred, despair, or dangerous tenderness and an inclination to sensuality. The best best remedy against such assaults is prayer, accompanied by true humility, self-disdain, confidence in God, the protection of Mary, the use of the sacramentals, and obedience to one's director, from whom none of these things should be hidden.

The other part of that footnote about forcing upon the person the demon's own sentiments can refer to the ability to manipulate our emotions in such a way as to impose the demon's "emotion" upon us (e.g., fear, hatred), The demon's fears, hatred, despair, etc. can be reflected through us. Since we feel these things we believe the emotion is ours.

God Bless,
Bro. Ignatius Mary