I had this exact problem, at the same point in my progress at judo.
Things that didn't work for me
I tried doing uchikomi slowly and deliberately. This usually ended up with me hunched over in a full squat, unbalanced, unable to stand back up with the throw, without any kuzushi applied to my uke.
I tried uchikomi for speed, whipping into each rep. Doing ogoshi or seoinage fast ended up one of two ways: not bending my knees at all (and thereby ingraining a bad habit), or bending my knees too much and ending up like the slow and deliberate reps (just faster).
I tried doing more reps, not really fast and not really slow. This just made me tired, and increased the number of bad reps I got in. Remember, "practice makes perfect" is a pernicious lie. It's "perfect practice makes perfect."
I tried doing drop seoinage and makikomi ("wrapping", falling onto one's partner) variations during randori. This worked...a little...sometimes...against weak players...in randori but not shiai. It didn't develop my technique, and was only a temporary poultice.
Things that did work for me
Squats. Heavy. Barbell. Squats. A bunch of other strength work (deadlifts! blessed deadlifts, plus power cleans, and presses and chin-ups) helped as well. After I was able to squat a barbell weighing as much as I did multiple times for reps, with ease, going back and putting in my uchikomi and nagekomi and moving nagekomi reps helped tremendously. I still have a long way to go, but the difference was like night and day.
Before, it was a question of collapsing in the middle of the throw, or not being able to bend my knees, or being too tired to continue bending my knees after twenty reps. Now, it's a matter of ingraining good reflexes and instincts.
Technique requires physicality. Technique can overcome athletic people's attributes, but no technique can be executed without a minimum of strength.
I was weak. Many people who go into martial arts are weak. That's okay. The problem occurs when we tell these people that just training martial arts will fix the issue, that somehow strength will arise naturally, and that technique will always eventually overcome strength. That's certainly true for some people, but it's certainly not true for others of us. (Many men who got strong by training hard and frequently during their teenage years say this. They rode the wave of testosterone.)
The less genetically blessed need strength- and power-specific training. Barbells are by far the best implement known to man to achieve those two tasks. Bodyweight training, kettlebells, and dumbbells are all great tools too, but the barbell is king for getting stronger and more explosive as fast as possible.
I agree that you need to work on endurance and technique. However, those get plenty of work in class. Strength does not. Furthermore, endurance is dependent on strength:
In the absence of developed strength, strength training always improves work capacity [and endurance -Dave] by reducing the relative intensity of repetitive tasks. (Mark Rippetoe in his Starting Strength forum)
Keep doing randori, keep going to class frequently, keep developing perfect technique and posture, keep studying timing and kuzushi. But outside the dojo, get a copy of Rippetoe and Kilgore's Starting Strength, a barbell, and a squat rack, and start lifting. (The program they recommend--three days a week without other exercise--is not appropriate for you, but the instruction and advice is top-notch.)
Greg Everett's books would also be a good choice; his Olympic Lifting for Sports is probably exactly what you need, but having only read his more general Olympic Lifting for Coaches and Athletes, I can't yet recommend it with full confidence.
Gant Grimes' lifting program for judoka is based on Rippetoe's, and might be appropriate:
Here is a two-day per week lifting program. If you work hard on this
for 6-12 months, you will be stronger than 95% of the people who spout
silly things on this forum (the other 5% have already done something
similar to this). Notation is sets x reps (3x5 is 3 sets of 5 reps).
A: Power clean 5x2, Squat 3x5, Bench 3x5, Chin 3x10-15
B: Power clean 5x2, Squat 3x5 (or front squat 5x3), Press 3x5, Deadlift 1x5
- You do this. That's it. I'd recommend another day or two of agility work, complexes, and sprints/prowler work, but that's another topic.
- You add a bit of weight to powercleans each week (not every day). It should feel a little lighter on B day. This is practice day.
- You can back squat both days if you want (that's what I recommend starting out). If you feel tweaked, if deadlifting is hard after
squatting, or if you just want to front squat, then you can alternate.
- If you can do 3x15 dead hang chins, then you need to add weight to keep the reps between 10 and 15.
- If you have extra time at the end, do farmers walks. Great ROI.
- All work sets sets are "sets across" (same weight for each set). Do 3-4 warmup sets (always start with the empty bar) to get there.
- Add ten pounds per week to squat, five pounds per week to your presses, and 5 or fewer pounds per week to power clean.
- Once the weights feel heavy the gains slow, work for 3 weeks and deload for 1 week. E.g. for squats: week 1 405x5x3 week 2 415x5x3
week 3 425x5x3 week 4 225x5x3 (or go play soccer) week 5 435x5x3 (or
425x5x3 if you need to) etc.
- When you record your workouts, it is weight x reps x sets. I'm not sure why this is, but it is.