Why do you say that you doubt you have it? Think of specific instances that they (they = doctors) are pointing to and saying that this is your definite diagnosis. If you doubt it in any way I encourage you to get a second (or third) opinion.

There were a LOT of things, looking back on it, that pointed to a definite bipolar diagnosis in my case. I could be depressed and suicidal for months. Alternatively I could be crashing the bank account and maxxing out credit cards and sleeping with men that weren't my husband.

It's just my opinion, but I think if you're diagnosed bipolar you probably realize that something is the matter in the first place. You might FEAR the diagnosis or WORRY about what's next, but I don't know if you'd outright DOUBT it. I knew I was. When I was diagnosed I was relieved, not doubtful.

Try keeping a mood chart or a journal. That's the most helpful I've found. You might not realize you're in a depressed or manic state until it's already over.