Worse still, my mind just isn't built for chess - on the mats or off.
I'm at purple, I suck at chess. Certainly, at white, I never was strategic. Nor at blue. I doubt I'll be progressing to brown without being more strategic. What I'm trying to say is, of all the things to let get you down, don't let this be one. It is entirely irrelevant.
Does anyone else find that jiu-jitsu is no guarantee of being in good shape?
Yes. 100%. When I started BJJ, I was 23/24 yrs old in a job where I was on my feet all day. I got into 'jiu jitsu shape' easily. A 29, in a sedentary job, I am in an uphill battle in comparison. I've added walking to work, morning squats/pullups/pushups...just to be able to attend drill class 4-5 days a week rather than 3. I currently am not rolling (attempting to work up to it). You are very far from alone here.
the techniques just aren't sticking.
I get this too. I pick it up easily though...but then it just doesn't really stay. I am mentally drained from work, when I'm not physically drained from under-recovery. Half my drill partners are exasperated. If I were at a different gym with different people, they'd get mean.
I'm actually depressed by the situation because it seems so hopeless.
I've been there too. I don't know you, so if you are the dumbest, weakest, slowest person in your gym I wouldn't know and I won't try and blow smoke up your ass.
But I will tell you this: at 120lbs and female, with 5 years of training, 29yrs old, and a fitness slump I am past delusions about my capabilities. Any person that is going to attack and hurt me* isn't the sort of person I'll ever be able to fight off. BJJ offers me 0 self defense benefit pragmatically speaking.
So why the hell am I even trying when (like you) I am slaughtered by everyone who trains (including every single last female regardless of size and mostly regardless of belt level), have little chance with people who don't (sitting here at the 10th percentile for height & weight), make my rank look awful, and live in exhaustion for no clear benefit?
I decided that, I don't want to live my life being the most pathetic version of myself, but the best version. I read. I play an instrument. I sing. I try and do well in work. I try and do right by my marriage. Life is struggle. I want to be the best version of myself in every way, and that includes physically. If I loose, I want to loose as least pathetically as possible.
I backed off the rolling for awhile. All of a sudden I am happier. My life is better. I attend drills, and try and go consistently. I am having fun again.
My advice to you is to take a couple steps back in order to go forward. Work on your fitness. Work on your sleep. Work on your health, strength, and happiness. Just drill in the mean time. Don't worry about what people will think or say, even you instructor. Get to a place where you are happy and making life progress. Let yourself miss this a little. Then try again. It'll be better next time around.
The dumbest guy I ever met was a brown belt. All you have to do is just not quit.