We just taught our 5.5 year old ro ride, but she was one of the youngest on the block. The other kids have learned closer to 7. She had a 12 inch bike that she had rode with training wheels for a couple years. We took them off and struggled since the spring to train her. I think we had success for a few reasons. The older girls on the block left her in the dust. We also took her to the track at the elementary school - far fewer distractions, safer with no roads, nice and wide, and no one to watch her so she didn't feel embarrassed. Finally, we took a good cop bad cop approach. My husband would work with her and she would complain "don't let go" and "I'm done, I want to leave" after 5 minutes. So he was good cop, and I said, "Will you do it with Mommy?" She thought I would be a pushover. But I dug in and said, "stop wasting time and just pedal." Also a little reminding her tha tshe can't ride with her friends - Once the training wheels were off, they weren't going back on.
They do have to get a point where they have a little bit of balance while you are holding them. Once they turn the corner with that, it is a matter of them allowing you to let go. That's where god cop bad cop really helped. But I think the trick was using the track. much easier than the sidewalk or street. Our daughter was really too big for a 12 inch bike and I think it wobbled a lot which didn't help her, but we told her if she rode on her own, we oculd go pick out a big girl bike - she actually fit a 20 inch. Another trick is that they need to look where they are going, not at their feet, and they have to pedal fast. IF they aren't going fast, it is hard to get balance. We also tried to appeal to her competative side - "look, you rode from here to here on your own - I bet you can go further next time" - you could even use chalk marks.
Personally, I think your daughter must ride with training wheels for at least a few hours/days to get the "feel" of the bike and balancing. My understanding is that you can adjust the training wheels higher to make them less dependent on them over time, although we didn't do this. If you persist and dedicate a few solid hours for a few days, you will see it all click. They go from riding 10 feet on their own to doing laps in a few minutes. Good luck!