It appears to me she is speaking of concupiscence and material things and our desires for them, such as desire for food, drink, etc.
And then she is speaking of our desire for spiritual things, which comes especially once we have tasted the supernatural life as Christians.
But we desire the latter in our own way, rather than in God's way, ie. our own timing, place, amount, etc.
When God knows best how and when to console us and how and when to let us journey unsupported by sweet things so as to show and gain virtue.
In her letters she talks about patience and surrender to God's Providence in all. For example, in another letter yet to be shared, to a lady whose only daughter died she wrote:
'It seems that God is calling you to great perfection. And I perceive it by this, that He takes away from you every tie that might hinder it in you. For as I have heard, it seems that He has called to Himself your daughter, who was your last tie with the outer world. For which thing I am deeply content, with a holy compassion, that God should have set you free, and taken her from her labours. Now then, I want that you should wholly destroy your own will, that it may cling to nothing but Christ crucified.'
And..
'Dearest daughter in Christ sweet Jesus : I Catherine, servant and slave of the servants of Jesus Christ, write to you in His Precious Blood, with the desire to see you established in true patience, since I consider that without patience we cannot please God.'
Thanks I had no idea what she was going on about either! Did you study these texts in a class or you just naturally have a facility for medieval texts? I wish there were Cliff Notes on these texts and its not only these spiritual ones I find difficult. I tried reading some of the Stoic philosophers and encountered the same problem! In fact I did find a study guide for Boethius' "The Consolation of Philosophy" which really helped. Ever read that one?