If an employee is driving you and your partners nuts (and your sanity is important to the overall success of the business), you have probably answered your own question--intellectually, that is. On a more human level, you are still trying to determine what to do; so I will dwell on that aspect of your question.

A good rule of thumb with any employee who is just getting to know how things work in a new job environment is that he or she is allowed to make each mistake ONCE. And that is a good thing to impart to new employees: Everyone makes mistakes. The important thing is to learn from each mistake and not repeat it.

However, all the transgressions that you mention that this employee has made are related to a general lack of good organization and a significant lack of ability to plan and/or manage her time. That is, she does not know how to get and keep herself organized enough to be effective, on time, considerate of the time of others, and careful of protecting company property, etc.

These are generally signs of immaturity, as well. You are not her parent. You are her superior in a business situation.

Many creative people seem incapable of planning or managing their time, but most can improve to a tolerable level if their shortcomings are
identified and they are given a chance to change and coached along the
way.

I recommend that you document the many mishaps she has experienced or caused to others in the past few months. Don't go back too far, or it may appear that you are reaching for issues. Point out how each instance has cost the company time, money or both, inconvenienced others, caused confusion and so forth. Tell her that regardless of the good she does on the bottom line, the chaos cannot continue. Ask her why she thinks these instances occurred. Give her a chance to talk. In some cases, she may be able to cite mitigating circumstances. Also, she may admit that she has had these problems arise in other jobs, too.

Regardless, suggest to her that she develop some of the vital skills that she is lacking through finding and pursuing training to become more organized. There are great time-management courses, seminars, online training sites, etc. If she embraces the need to change to succeed, you should be able to discern it from her demeanor and from whether she follows through to identify how she will address her training need and work/lifestyle changes needed.

If you ask her a week later what she has done to find and commence training and the answer is "nothing," you will have your answer. My dad always said, "You can't help anyone who will not help himself/herself." He was a wise man.