Bipolar Disorder has become over-diagnosed even though the symptoms often fit PTSD better. I've been diagnosed with both, and more.
My advice is to deal with bipolar symptoms with meds your doc carefully monitors. But try not to associate bipolar disorder with your eating disorder. Check the DSM-IV for symptoms and focus on the therapies available.
And there are many meds for bipolar. If you take one, pay close attention to the benefits vs. side effects. Many cause weight gain. Many actually make depression worse.
I took them all, but finally decided only to take lamotrigine (Lamictal) to dampen my mania. I had a shrink who was furious I wouldn't take Lithium, which she prescribed at our first visit. She didn't know shit about me and had no skills in caring for my moods and actions. Lithium almost killed me. It caused tremors so bad I couldn't write. Not being able to write? THAT would kill me.
You won't be denied because of bipolar. Approval is based on many things, but I think BMI is most relied on by docs because it can be measured.
I hope you have a good shrink who listens to you. Have a back-up doc who can also track your health. We are complicated machines, and fortunately, our functions can be measured.
But meds can mask symptoms, and clash with other substances. You're unique. It might take time to figure you out. Docs need your feedback. They won't tell you what to expect because they don't know.
Bipolar disorder can be a gift. The greatest artists, writers, actors, musicians, etc. were diagnosed bipolar. Check it out on the internet. One great artist from the 1940s who responded well to lithium said he'd spent his life in sorrow for so long, only to find a few grains of lithium saved him.
There's no cure-all, but if you are bipolar, it's likely some kind of medication will help. Keep an open dialog with a medical professional until you find what works for you.
And as you lose weight, your dosage will probably need tweaking. It may take a year or more to find balance, and losing weight may improve your mental state, so focus on it. WLS is often a big part of the cure.