The uneasiness that you experience at prayer, together with your anxiety to find a subject that can captivate and satisfy your mind, is in itself enough to prevent you from finding what you seek. When we are too intent in our search for something, we can look at it a hundred times without seeing it.
Such useless anxiety can only result in weariness of mind which in turn produces this coldness and numbness in your soul. I don't know what remedies you should apply, but I do think you would gain a great deal if you could keep from being so anxious, for that is one of the greatest obstacles to devotion and real virtue. It pretends to incite us to good, but all it does is cool our ardor; it makes us run, only to have us stumble. That's why we have to be on guard against it at all times, especially during prayer.
And to help you be vigilant in this, remind yourself that the graces and benefits of prayer are not like water welling up from the earth, but more like water coming down from heaven; therefore all our efforts cannot produce them, though it is true that we must ready ourselves to receive them with great care, yet humbly and peacefully. We must keep our hearts open and wait for the heavenly dew to fall. Never forget to carry this thought with you to prayer: in prayer we approach God and place ourselves in his presence for two reasons.
The first is to render to God the honor and praise we owe him, and this can be done without his speaking to us or our speaking to him. We can fulfill this duty by acknowledging that he is our God and we, his lowly creatures, and by remaining before him, prostrate in spirit, awaiting his orders. How many courtiers there are who go into the presence of the king over and over again, not to speak to him or listen to him speak, but just to be seen by him and to indicate by their regular appearance that they are his servants! This aim we have in presenting ourselves before God simply to demonstrate and prove our willingness and gratitude to be in his service is excellent, very holy and very pure, and, therefore, a mark of great perfection.
The second reason why we present ourselves before God is to speak to him and to hear him speak to us through inspirations and the inner stirrings of our heart. Ordinarily, we take great delight in doing this because it is very beneficial for us to speak to such a great Lord; and when he answers us, he pours out much balm and precious ointment, and in this way fills our soul with tremendous consolation.
So, Mademoiselle, my dear daughter (since this is how you want me to address you), one or other of these two benefits can never be absent from your prayer. If we are able to speak to our Lord, let us do so; let us praise him, pray to him, listen to him. If we are unable to speak because our voice fails us, let us, nevertheless, stay in the hall of the king and bow down before him; he will see us there, will graciously accept our patience, and look with favor on our silence. Another time we will be very surprised when he takes us by the hand, chats with us, and walks with us up and down in his garden of prayer; and even if he never does this, let us be satisfied that it is our duty to be in his entourage and that it is a great grace and a still greater honor that he allows us to be in his presence. In this way, we won't be overeager to speak to him because this other manner of being near him is no less useful to us and, in fact, may be more so, although not so much to our taste.
So when you come before the Lord, talk to him if you can; if you can't just stay there, let yourself be seen, and don't try too hard to do anything else.
"Excerpts from Letters to Persons in Religion: a Letter to Mademoiselle de Soulfour by St. Francis de Sales