I've never had an orchiectomy, but I was diagnosed with hypogonadism nearly ten years ago. Back then my testosterone levels were 150 ng/dL. That's about half of the normal low range for T (300-100 n/dL). Now without any HRT, my T levels are not even detectable. I think what happened to me is similar to what you will experience.
Here's what my symptoms were: Low energy, even after a good night's sleep if I sat down in a comfortable chair in the late afternoon, I would doze off quickly. I wasn't really depressed, but wasn't excited or interested in much else either.
In short, I felt about 10-20 years older than my actual age.
Well, I took testosterone injections for a couple of years, but that sort of backfired. My body evidently has an abundance of the enzyme aromatase. Aromatase converts testosterone to estradiol. We all know what estradiol does.
After your orchiectomy you will need either estrogen or testosterone for most of your remaining life. I hope you will be content being a woman, because you will find it very difficult to remain male after that surgery. In my case taking testosterone only made me a big strong woman with big strong boobs. (I measure 52" across the bust).
Best wishes,
Randi