Why can't you seek help?

You need to be diagnosed by a psychiatrist. Self diagnosing is not helpful in the least, and dangerous at the worst. Please go to your doctor and ask for help if you feel you have something wrong. There could be physical reasons that need to be ruled out before a mental illness can be confirmed. You can't know you are bipolar just from reading or taking a test online. You need to have a full check up and diagnostic interview. It can take years to have a correct diagnosis.

I wouldn't mention it to anyone in a relationship because you don't know for sure that you are bipolar, and you will end up sabotaging the relationship regardless because you will be self fulfilling your prophecy.

Melatonin will not effect your mental state, it should help you have a more restful sleep. Melatonin is made naturally in our bodies, if anything the only time you will experience side effects is if you suddenly stop taking it- you will have worse insomnia.