I'd love to hear how others handled the doubt while trying to accept poly. It feels like Im just about to step off a cliff...
I know, it does feel like being about to step off a cliff! I think the way to handle the doubt is to accept the doubt itself, because it is normal. You won't be able to accept poly overnight; it will take time. Three months ago when I first found out about my husband's girlfriend I thought there is no way in the world this can possibly work. Even one month ago I was very much doubting it, and I still have a lot of doubts, but as time goes on I get more and more used to the idea, and even excited about it.
Just keep communicating with both of them and let time do the rest.