I don't totally agree with the consensus of this cheating not being at all attributed to the Bipolar Disorder. It is commonly known among doctors/psychiatrists/scientists that a symptom of mania is indeed impulsivity and this can include sexual recklessness.

I speak from personal experience when I say that mania can make people do things that they regret at a later point in their life. When I was given a job at the end of March this year, it triggered a manic episode. I felt on top of the world, made grandiose plans and felt completely unattached from my emotions - incapable of feeling any negative emotion at all; only happiness and a feeling of self-worth. The following month, during the worst of this episode, I cheated on my boyfriend of one year. It was partly inspired by severe paranoia, and I later thought I was in love with this boy I cheated with.

After a couple of weeks, I crashed. I had cycled into a mixed episode. I got into an argument with my boyfriend and I blurted it out that I had cheated. I was full of anger and paranoid delusions. I told him he can't talk because he himself was cheating (this turned out to be an incorrect assumption caused by paranoia). When I told him about it and for a few days after, I couldn't convey guilt. I felt horrible, but I couldn't cry. I couldn't feel sad, nor happy. I just felt very angry.

Then came the depression. I felt so horrible for what I did. I cried all day, every day, for weeks. I showered my boyfriend with gifts and apologies; wrote him poems and it was a tough month. Very tough.

Now, though, we are stronger than ever. I love him more than anything in this world, though I always have. Now I just know I can never do what I did again. The guilt still sometimes consumes me.

I've used my own experience as an example, as when I was manic I was so impulsive in my actions and just didn't think about the consequences or the feelings of others. I just didn't think ANYTHING through and I spent £3000+ in less than 3 weeks on complete rubbish. The reality of what I did only hit me after the manic episode had passed.

However, as your girlfriend has been cheating for four months and from what else you have said, I'm not totally convinced it's due to a manic episode (or even a hypomanic episode). It seems to me as though its a personality thing and I really think, if you're both dedicated to the relationship, some form of couple counselling should be considered - along with the appropriate help for your girlfriend's Bipolar. Like I said, I'm not totally convinced this isn't just her personality causing issues.

You need to give yourself a little care and respite to think things through. I feel for you and I think you have the strength to overcome this.

Good Luck
RB.