What 'kind' of trainers have you asked for apprentice opportunities?
There are 'general' trainers that start colts, ride and school trail horses, Mounted Shooting horses, etc, and there are 'event specialists' that show in one event or kind of event like reining trainers, Western Pleasure trainers, Cutting trainers, halter trainers, show and sale fitters, Hunter Under Saddle trainers, and the list is endless. They are going to want an apprentice to have a lot of experience in that venue.
There are regional trainers that take in a lot of local or non-show types of horses. If they show at all, it will be in 'open' shows and small local shows. These are the only things you will get from any of the Clinicians. None of the big-name clinicians are able to train or take horses to a high level in any event. The good event trainers only do clinics in that particular event. They are not people like CA or PP or JL.
If you are interested in a particular event, then you need to aim toward that event in your riding. Is there a particular direction you want to go? Have you put together a series of videos showing how well you ride? If you have, start sending them out and ask what kind of apprentice positions might be available for someone that rides at your level. If you have shown, show any videos you may have of you showing. Even if you want to expand to a different venue, show videos show raw talent and ability.