People who are bi-polar will relapse. They will get worse, they will make your life a living hell. Then they will get better until the cycle repeats itself.
Do yourself a favor, get a copy of Wishful Drinking by Carrie Fisher if you want a good idea of what you could be dealing with in an ongoing basis when dating a bipolar person.
Yes folks, Princess Leia is a bi-polar sex/drug/alcohol addict with parental issues who had a gay friend die in bed next to her not too long ago. If you can see her perform the book on stage, it's even better.
"Well, then," the Cat went on, "you see a dog growls when it's angry, and wags its tail when it's pleased. Now I growl when I'm pleased, and wag my tail when I'm angry. Therefore I'm mad."