If you have assumed a character above your strength, you have both acted in this matter in an unbecoming way, and you have neglected that which you might have fulfilled.
The concept of duty is important in Stoicism. While a Stoic should focus their desire on things within their control (ie, philosophy), they should also strive to do good in the world. If successful in their endeavour, they have made the world better and can take joy from that; if not their philosophy gives them some immunity to the costs, such as loss of wealth and reputation. Taking it further, being unsuccessful is an opportunity for a Stoic to practice their philosophy in anger.
This concept of duty, combined with Epictetus' emphasis on working in accordance with nature, is at the heart of this quote. You have a duty to do good; if you disregard 'nature' (your talent, knowledge etc) you jeopardise your ability to perform this duty.
It's also worth thinking about what would make you "assume a character above your strength". To me, this sounds like greed, vanity or power-addiction influencing your decision making. To phrase it more positively, the quote above could become
Focusing on things that are within your control will give you clarity regarding your strengths. This will allow you to contribute to society in the most effective way.