All this is pretty much par for the course when coming to terms with polyamory, especially for monos I'm afraid. Initial successes and glimpses of the positives can lull us into a false sense of being more together than we really are. It's a process and it helped me to look at it as being a period of ramped up self-growth.
I started posting on here and also started my own blog (link in signature). You might find reading about some of my struggles and insights useful. It does get easier but you have to be patient and be prepared for a few growing pains.
It sounds like what you are going through at the moment is a grieving process for the marriage you had before polyamory. It won't be the same and I don't know if you can successfully go back, but you can aim for a relationship moving forward that is wonderful and still has you growing old together in it.
Sorry I can't remember how old this other relationship is but it sounds like there could be some NRE going on. I think there are some threads tagged on how to deal with that.
It will get easier, take care.