My mind is saying scary things to me?

Two years ago I was diagnosed with bipolar disorder. I was planning on killing myself. I had a real plan and a time set in my mind, how I was going to do it and everything. It turned out I didn't do it. I found something that kept me happy, or at least made my life bearable for awhile. I continued to live, I didn't try to kill myself, I was a good girl and kept my psychiatrist and therapist appointments and took my meds. I had one hypomanic and one depressive episode since the time I had seriously planned out killing myself. Overall, I've been doing okay. Some things in my life have happened that I'm surprised haven't made me really depressed (my closest and only true friend was gang d a couple of weeks ago and he is in his own little world now - he barely trusts me anymore). So today I was feeling ok like eveyday for the past several weeks, and it was a relatively warm day, and sunny. I was driving home, and out of nowhere, my mind just started wondering why I hadn't done it, why I hadn't killed myself when I had been so close. And it really scared me, because I haven't reconsidered even the reasons for killing myself since I decided not to. I guess I'm so sick of being unhappy. And there are things going on right now in my life that are good, but then something else comes up that just shoots them down. I thought maybe my meds needed to be adjusted, but i've kept going back to my pdoc and she doesn't really do anything to help. And I'm sort of taking care of my friend right now and I don't know what is going to happen with him...I don't want to give up but my mind screams "there is nothing"