It's interesting how much better I feel just since writing this initial post and the kind response from River, reading both my own words and his replies again, I had a sudden insight ~
I've been looking at this experience in my life all backwards, feeling like something is wrong with me for feeling the way I've been feeling, and struggling with the fear I've been struggling with. It would help me more to ask "Why am I feeling this way?" and "Why do I need this experience right now?" and "How is this feeling, this experience with fear, meant to be a source of healing and growth for me?" Some part or parts of me deep inside have been hurt deeply, probably long ago, and this fear experience is an opportunity to understand myself better, love myself more, and find healing and greater wholeness, and to learn new deeper levels of strength in myself, and trust in those who love me.
But I need to be more patient and compassionate with myself, to follow Will's example of unconditional love for me, and give that to myself as well... this issue will take time to heal, and beating myself up for it will only make it worse and take longer. If I can give myself time, safe space for talking and exploring the feelings and what is behind those feelings, and treat my own heart with the same love I treat others... then I can trust that I can heal the fear behind my insecurity.