I feel your pain on this.
My husband and I set a plan and work on it. then things change and he acts as if I am horrible for being who I am and wanting to find a poly partner(s)
However: here's OUR biggest issue: He wants to find someone to fit into a perfect mold.. and one of the BIGGEST Don'ts that I have read is dont try to form relationships that fit into a pre-defined mold, and thats exactly what he wants. (and thats not what I have found)
I agree with the therapy part... but I also think that he needs to be honest with you about his needs (does he need to know 10000000% that you are not trying to replace him? and how does he want you to show that??)
Sending HUGE hugs your way and positive thoughts that you are able to break his barriers and work thru this in a healthy way for the both of you.