With a good instructor, maybe we would all solo in less than a specified number of hours. But a range of factors come into it (weather, airfield, instructor, student, lesson frequency, ....) What is most important is not the hours but the reasons.
Some suggestions:
- Stick with one instructor. They will be able to monitor your progress better and understand which areas are going well and which need work. Everyone has their own style and if you change instructors you are going to get lots of different advice.
- Ask for a list of skills required to solo and check off the ones that are complete. Then make sure that future lessons are focussed on the areas you still need to work on.
- Move on with other skills like navigation. It will improve your confidence in your flying (there's more than just circuits) and take the pressure off. Going around and around in the circuit can be demoralising.
- After a lesson, ask the instructor to spend time with you going over what needs to be improved and what to prepare for next time. That way you can make the best use of your lessons.
Stick with it, and good luck. I started flying lessons 15 years ago, was in a similar situation to you, and gave it up. Two years ago I went back to it, had a completely different experience (even if I was not the speediest to solo!) and have my PPL.