My reasoning is that you feel a void, and express the longing for a sence of fulfillment. This is the sort of language you used. This is the "unrecognised spiritual need" and is exactly what is being addressed in the letter. The answer, at least as far as Jung is concerned, is a "higher understanding". Note the three ways that this can be achived. Only one of which (at least to my way of understanding of this text) is strictly a "religious" path. IMO this letter is quite profound. What Jung is getting at is, as he says, easily misunderstood.
You expressed that you would like to be more spiritual. OK, do it. Look for some of those spiritual understandings, and don't confuse spiritual with religious. You have to seek it. IMO you'll find what you need in direct proportion to your efforts.
All the best to you.