Is there any decorous way to acknowledge my failure to do so over the past few years and to express my regret? Or would that simply be self-indulgence, detracting from my message of gratitude?

However, while I am always -- always -- effusively grateful and do my best to show her the same considerations, I have never sent a thank-you note. I was simply never taught, and am saddened to think I missed out on so many wonderful opportunities to do so.