It sounds like what happens is that you, with the best will in the world, start out trying to practice the scales slowly, but then muscle memory and enthusiasm take over and you speed up and get sloppier. I'd suggest giving your brain something to focus on while working on slow scales. There are lots of options. Pay attention to the sensation of your fingers lifting. Try to time the lifting of one finger with the fall of the next so exactly that you can trick your brain into thinking that it's the lifting finger that's producing the sound. Focus on getting the volume exactly the same between all fingers and the thumb. Pay attention to one particular finger as you play the scale, making sure it is relaxed and plays exactly on time, without thinking about the others. Pay close attention to the sensations in your hand; how relaxed can it be, how does it feel when you play a scale smoothly and evenly. Focus on the smoothness of passing the thumb. Play very, very slowly, sometimes and listen to everything about the sound, volume, decay, connection to the next note.
And it seems obvious to me, but do this all hands separate for a long time. That disrupts muscle memory and it should help you hear more clearly what's going on.
Those are just suggestions. Anything you do to engage your mind and keep interested in what's happening in slow scales will hep keep you from just "blagging" away. Even if you cannot afford a regular teacher, having someone just check that your basic technique, posture, arm, wrist and hand position are OK would be good, too.