At some point in our Aikido journey we all have challenges with our ukemi. Most often it comes about from a bad roll, a rough nage or we just seem to have hit a wall. Whilst the temptation is to see this as nage's problem or some external reason the simple fact is our ukemi needs to continually develop alongside our Aikido expertise. Ukemi and Aikido go hand-in-hand with making progress.
Negative experiences can often outweigh positive ones. So its important to get back to the positive experiences as quickly as you can. Here are some pointers:
1. Solo ukemi practice is important, as is feedback on it. Just a few rolls before class, at a level that you are comfortable with, will lay the foundations for progress. Get a senior or an instructor to give some pointers.
2. Don't stop. Once you decide mentally that something is too difficult, and stop doing it, its going to be really hard to improve. The mind leads the body so you need to stay engaged mentally. Find a way to participate as fully as you can - even if means taking a different ukemi (e.g. a backward sit down roll can be used for many forward projections)
3. Find the level of training where the experience is positive and rebuild from there. Slow your attacks down to match your level of skill at ukemi. Slow your ukemi down until you are comfortable taking it. (e.g. for a forward roll, drop to your knees first and go from there.)
4. Ask around. You will find someone else that has had the same barrier as you. Find out how they got around it.
5. Dojos are founded on the sempai-kohai relationship (but on a bigger scale), find a senior who might be willing to work with you before / after class or in a park. Rebuild your ukemi with someone you trust as a nage.
6. Remove psychological barriers. Double-up or triple-up on mat thickness until you are comfortable. Use a big crash mat, find some trick that lets you approach ukemi without fear.
7. Take it on the chin, leave your ego at the door, if you get advice look for truth or the grain of truth in it. Public castigation in the dojo of another style helped me see outside of the box and fix up something fundamentally wrong in my ukemi.
8. Get involved in the beginners' program. You can learn a lot about something by having to teach it. One caveat: be sure you teach by example not just by talking about it.
9. Finally if you need a clean slate on which to start again, free from expectations, visit another dojo. (Your teacher may be able to recommend one.) Consider strapping on a white belt and work at your own pace.