Thank you for such a thoughtful letter. I can’t tell if you have depression on the basis of a letter. I can tell you that what you report is reason for concern. Self-harm is a serious matter. Feeling so sad that your only way to channel it is by hurting yourself is a terrible way to live.
It’s notable that you are known for being generally cheerful but then you have these, out of the blue, bouts of depression. If we were talking, I’d first be checking for these two things:
1) Are you getting enough regular sleep? A sleep disorder sometimes is a factor in descriptions like yours.
2) I would then evaluate you for either depression or bipolar disorder. People with mood disorders experience mood shifts that can be unexpected and can vary enormously in intensity.
I strongly encourage you to seek out a mental health professionals to do a thorough evaluation. You’ve been dealing with this for at least 6 years. Although you’ve done a remarkable job restraining yourself from self-harm, the impulse is still there. You need the help and support of a professional to get to the root cause of your feelings and to help you decide on a treatment plan for managing them.
Please take yourself seriously. Love yourself enough to get the information you need to live a more carefree life.
I wish you well.