Re: slow progress
I can completely understand where the OP is coming from, however, for me, I embrace the chance to train with the new people who have not had a chance to 'learn how to take ukemi' or that do not adopt a 'proper' stance.
Training with others at 'my level' is certainly nice and I do learn a great deal from that training however I have found that for me (this may not apply to everyone) that training with beginners is greatly beneficial.
My Sensei has explained that given a chance to train with some one who does not know what they 'should be doing' (how to grab, not enough resistance, to much resistance, how to roll or fall etc..) is an excellent opportunity for me to practice developing my technique so that it actually does work, all the time.
This means that I have to do it slowly and I mean very slowly and thereby safe. It gives me time to do as others have suggested here, focus on the not only the little details of the technique but the overall 'feel' of it, so I can move as 'one' as best I can. If I can take the centre of a new person who has no idea what to expect and so does not comply/conform with that expectation, then I know I am much closer to doing it right.
Just like the OP, I have to get the technique done, let the my Uke 'feel' it on them first and then explain 'my understanding up to this point' on the second go through and then I usually help them perform the technique on me and provide just enough resistance to help them 'feel' some development in the technique.
My Sensei usually 'rewards' me afterwards by showing me more in depth explanations of the technique personally. A lot of personal training and my favorite part.. getting to practice my ukemi by attacking Sensei as fast as I can get up from the last technique while doing Jiyu-waza.
You have said you have no 'middle' speed and that your technique is unsuccessful at slow speeds and so you have to 'power through.'
I used to be the exact same way and had the same results. Even when I thought I was going 'slow' it was not nearly slow enough. My Sensei can do the technique very very slowly against much larger Uke's that I know to be very strong (Carpenter that works with all hand tools... really scary thick wrists, you get the idea.) and the techniques work, even at slow speeds.
So I am following that example. I believe that if you cannot make the technique work at slow and steady from start to finish (no stopping and starting, no 'jerking it') then it will not work at speed very well, mistakes will happen and, at speed, accidents will as well.
If all you do is train 'flowing' at speed or with just 'powering through' (using speed to compensate for not understanding the technique properly) ... when a strong, resistant, non-compliant, 'partner' gets a hold of you, chances are the technique will not even start and thus, fail.
Moral of the story is: There is always something to be learned by training with anyone... however it helps to know what you are trying to learn or you will miss it.
When training, embrace each perceived failure as understanding another way not to do it. Chip away at it enough, with enough help and guidance and the correct way with show itself to you.