I've found that sometimes the mind is the only thing holding you back. It's natural for the mind to think, "don't fall off." With that warning in place, the body gets apprehensive. It won't want to find that neutral balance point.

Try intentionally "wheely-ing" too far so you dismount off the back of the bike. Bike to a flat grassy area with your helmet. Moist ground is softer yet. If you have clipless pedals, don't wear your clipless shoes. You don't want to be hindered. You can even keep one foot off the pedals all together. Find an easy gear (not necessarily the easiest), and drive the pedal down while extending your arms and leaning back. Your free foot will catch you. If you did this correctly, you are now standing with your bike on its rear wheel. If not, hopefully you picked soft ground.

Once you get accustomed to going too far, combine the previous posts (squishing the front tire, etc...) and continue practicing.

BTW You want to practice with at least one finger hovering over your rear brake. If you unexpectedly go too far, touch your brake and your front wheel wheel will come back down. In the past, I've received weird looks from people. It's not every day they see a biker on his back with the bike in the air.