Not sure where this became a conversation about borderline as opposed to bipolar, but they are very different things.
I was diagnosed with bipolar as a teenager and I've since learned that it's a very misunderstood condition. Bipolar I refers to episodes of mania and depression, and rarely in a rapid cycling fashion.
Mania is not simply "feeling good", it's extremely disruptive. When I've been manic, I've stripped in public, accused people of poisoning me, flew to Florida (from fucking Australia) thinking I'd be recruited as an astronaut, bought cars, spent thousands of dollars of nonsense and spent days bouncing around bars and nightclubs without sleep.
As a working professional, I live in fear that I'll become manic on the job and do something to destroy my reputation and career. The strain this puts on personal relationships is incalculable.
As for depression, that is a far worse kettle of fish. I've seen patients become catatonically depressed after sky-scraping mania. One man I remember didn't leave his bed for months, smelt like a corpse and could barely speak/eat.
Depression isn't always "feelings of hopelessness", it can be extreme and debilitating.
My experience with depression following mania has seen me stay indoors for weeks on end, make attempts on my life and lose extraordinarily dangerous amounts of weight.
The only advice I have is this; bipolar I is an extremely difficult illness with which to live, and may never be well-controlled. If the good outweighs the bad, I wish you luck. Love isn't easy, so it's gotta be worth it :)