Thanks for your comments, everyone.
I think I have a basic fear of my partner not being straight with me. I'll talk to him now I've had a chance to think things over and now I've researched a little more into polyamory in general. I clearly have some issues, and they're mainly about trust.
One thing this has taught me is that the way people want to behave can change. It will do us good to have the "are we going in the same direction" conversation again, as it's a while since we did that.
I'll keep reading
Edit: a couple of you wondered why it had taken me so long of worrying about this. The answer is that we have had life problems happening one after the other, all intertwining and affecting our relationship, and we are only just beginning to untangle everything. Even though this has been bothering me that much, it has only just reached the top of the list!