Don't feel discouraged about your mistakes in some pieces. Stay the course and you will find that the progress is usually just around the corner. I understand that it might be difficult to see the progress for yourself because you are measuring yourself everyday.
There is one thing that is used in measurements and testings of all kinds in many educational systems which you can take advantage of. Take an unfamiliar piece and play it through just once. Record yourself playing this piece. You will make many mistakes, of course. Too many, actually. And that's the point. Then leave this piece alone and practice your own compositions which you normally practice daily. After several months you come back to that new piece and play it once again (and record it). Then you'll see what happens.
The thing is, because you are practicing many different pieces now, your overall level improves and because of that your sight-reading abilities also improve gradually. So when you come back to this new piece after a few months, you will be able to sight-read this new piece at a higher level than before regardless of whether your practiced it or not. Does it make sense?
So that's why it is better to stay positive when you are noticing yourself making mistakes in your organ playing. Because you meet yourself practicing every day, you just can't appreciate how much you are progressing in reality. It is kind of similar if you meet a person and go away for a few months, come back and you notice how much he or she has changed. But if you spent that time with this person every day, you just couldn't notice many important changes that took place.
The same is with organ practice. You just have to give it time and test your progress after a few months to notice some tremendous changes in your abilities and technique. Just test the two different recordings of the same piece. And that's when it get's really inspirational and exciting.
Try this technique in your organ practice today. Test yourself and you'll be surprised about your true skills and progress.
By the way, do you want to learn my special powerful techniques which help me to master any piece of organ music up to 10 times faster? If so, download my free Organ Practice Guide.
Or if you really want to learn to play any organ composition at sight fluently and without mistakes while working only 15 minutes a day, check out my systematic master course in Organ Sight-Reading.