If I'm understanding your story correctly, you are a frustrated adult rider who:
1) started riding 3 years ago with 10 months missed
2) has owned 3 different horses
3) has ridden with 3 different trainers
4) has tried several different horses (on trial to buy or in lessons)
5) has ridden in at least 2 different disciplines
6) is now looking to try another discipline AND possibly anther breed of horse
7) gets bored if it's too easy
8) wants to be challenged
9) needs to build confidence as well as riding skills
and you want to know how long it will take you to be able to ride a horse properly.
No matter what discipline or breed of horse you choose to ride, it's what you do consistently that makes the difference
Choose one discipline. Find a trainer you like and who is experienced with teaching beginner adult riders. Stick with that trainer and dedicate yourself to learning that discipline.
Work through the 'boring' stuff as well as the challenging stuff.
When your trainer agrees that you are ready for your own horse (one that won't be too much for you and you won't be bored off in a few months or a couple of years), have him or her help you find the right horse.
Becoming a competent rider takes time - that means years - especially when you are starting out as an adult. How long it takes depends on:
1) your physical ability
2) your fitness level
3) your commitment and dedication
4) how often you ride and practice what you are learning in your lessons
Take your time and enjoy the process. There really is no 'destination' when it comes to riding. I have been riding for most of my life and am a professional trainer and coach. I am still learning and (I hope) improving.
The journey is worth it.