I wouldn't overthink it like that. The development of SSRI's were essentially a half-lucky diceroll that managed to, largely, be a surprisingly effective path to treating multiple psychiatric symptoms (anxiety, OCD, depression, anger and behavioral issues, etc). And they still don't "exactly" get how the second phase of therapeutic action actually works, because they can't easily trace or measure the effects past the first phase.

Back to what you're experiencing, probably somnolence? Or something in that general area of "unexcitable blahness" and feeling numb. There's a few possibilities, as it pertains to you: SSRI's might not be the optimal direction, the specific dosage might not be optimal, the effects from the specific manufacturer may not be optimal (see my other comment for further info), the drug itself (as opposed to another SSRI) may not be optimal. You "may" also find that you can reduce the side effects a bit if you take it at night instead of during the day.

I could "probably" help narrow things down on a few of these items, but it's still a bit of a crapshoot of trial and error. Which believe me, I have ranted to myself about quite a lot since having more recent issues. And for years I've been irritated how doctors don't simply utilize or provide more reliable diagnostic tools simply because it's not "convenient" (And there are tools that they basically use when they formulate the drugs. But the medical assumption has been that there'd be "little benefit" to using these to fine-tune treatment regiments)

Also, read my big ol' comment that I just barely posted for more info.