Crummy training sessions.

It happens. I know it sucks, but just forget about it and move on.

If it happens a few times in a row, consider taking a week off from training.

I read somewhere that, generally, out of 5 workouts: 2 will be good, 2 will be meh, and 1 will be bad. Of course, no 2 people are identical, but it helps to know going in that some workouts are just gonna suck.

Sometimes when I work a particular muscle group (like back or chest) that for me personally is a bit weak, I don't quite "feel pain" but it feels weak, and I struggle to move the weight/continue with the exercise. If I continue to push through I usually feel pain the next day. I'm not sure if this is what you're describing, that feeling of weakness in your muscles, not pain.
Anyway, my point is, you're not alone. It happens, next time will be easier.

Some people eat before a workout which can make them tired, but a couple of nuts won't do that.

Don't worry how much weight your pushing, everyone starts somewhere, check your ego at the door and do what is comfortable for you but still give you a challenge. Higher reps you sculpt, lower for strength, but if you start hearing some snapping better be careful, cause the last thing you want is to be injured.

Hell yeah! You ain't going no where with dehydrated muscles, today drink around 8 cups of water and try to keep your sodium down and don't drink much diretics(coffee, soda) before your workout. Tomorrow you should notice a difference. Also about the numbness, get checked out just incase,I'm no doctor, but it's better to be safe then sorry. It's hard for me to know exactly what's going on if I'm not there to see it. Also what works for me might not work for you. Just a bunch of trial and error till you get it right.

6-8 rep ranges don't usually cause much hypertrophy (bodybuilder's call it the pump)... so it's natural to not feel full in the muscle. Do dropsets! Increase volume, and you'll feel that pump, up the rep ranges to 8-12 or more for hypertrophy.

Dread the idea of working out, then maybe you have adrenal fatigue, lots and lots of high-intensity training && long-term caloric deficits, and/or stimulants (coffee, tea, redbulls, pre-workouts), if you're accustomed to taking these, and suddenly you go cold turkey, I think the body forgets how to produce adrenaline naturally. Easy way to find out is, drink a cup of black coffee before hitting the gym, if you feel good, then you can try going a week without stimulants, AND stop your workout sets 1-2 reps before failing, but add in more sets to maintain volume, drop weights if you have to. Give it a week or two, and you should recover from adrenal fatigue.

The days, when you feel crap, usually, meals leading up to the workout aren't that good... keep track of "how you feel" during workouts, and log those too, then you know what meals work and what meals don't. Usually, easy digestable carbs are good, i.e. sweet potato and white rice, NO WHEAT, some protein, and some good fats, and lots of water, sip on it slowly, don't chug down a litre at once.

Strive to train with incredible focus and intensity, that way, you'll always, ALWAYS, want to go back.