One of the things to understand is that the term "high priestess" is a job title.
In Wicca, all practitioners are their own priest or priestess because we serve our Gods with no need for an intermediary. That is true whether one is a solitary or in a coven, whether they are new to the path or have walked it for many years.
Now, the term "high priestess" is given to that priestess who is in charge of a coven. The very fact that you use the term "high priestess" suggests that there must be more than one priestess in a group...otherwise the term becomes meaningless. So someone who is solely a solitary practitioner is never a high priestess because by definition there are no other priestesses over whom she has some authority.
If you truly would like to be a high priestess, then it is likely that you are going to have to find a coven with which to join and study. Then in time you might become a high priestess.
Being a high priestess is an awesome responsibility since you then take on the responsibility for leading a group, counseling group members, teaching students, leading rituals and all of the other tasks inherent in the position. I never recommend that anyone try to take on the job unless they feel a real calling to serve the Gods in what is essentially a clergy capacity.
And don't expect to reach that point quickly either if you study with a Tradition. I know that in my own Trad we expect that it will take 5-7 years of work and training for someone to reach the point where they are ready to lead their own group