When should a Mormon talk to their Bishop about a misdeed as opposed to repenting "on their own"?
There is no set line. That's why it's important to keep studying the gospel, to get personal answers to your questions as you study, pray, and meditate.
One common answer to "where is the line" is, "if you are asking yourself that question, you should talk to your bishop." There is a lot of truth to that.
There are some good answers to common questions at the Church youth website. On that page you will find this issue discussed. To me, it's important to know that there are many people you can ask if you have questions about mistakes you've made:
Parents or trusted family members
Friends in the church
Church members, such as your seminary teacher, or a member of the bishopric, etc.