I feel that perhaps I should clarify abit. When he made suggestions about changes in myself, I don't think they were that serious and logically, I know that he wasn't thinking of her when he said them. It's just that they were all very similar to her and because of my existing insecurities I associated them that way.
As for comparing me to her and talking about her a lot, I did evetually tell him about it and how it bothered me over half a year back and he apologised and said he never meant to make me feel second class.
It's simply that when he's excited about something he can't help but express it (it's actually one of the things I really love about him). When we got together, he had recently ended a long term monogamous relationship due to his guilt that he had pursued other partners (including her). When we agreed to enter an open relationship he was just so excited that he would be able to see her without feeling guilty about it. So he talked about her a lot and made the comparisons.
I know and understand why he did it but because I was so new to open relationships, it made me insecure and some of that insecurity is yet to fade.
However, in good news, I no longer feel uncomfortable about the idea of them being together! (Although sods law dictated that about the moment I came to this revelation, it turned out that, whilst loving him, she doesn't want to engage in a relationship after all).
After hearing your advice and doing some self-searching I realised the reasons why it scared me so much:
1) It was unexpected. Me and her, we've become friends, and a few months back she had expressed to me that she didn't feel like she could engage in a relationship or anything sexual with him again. So when she told him she loved him it was a shock. I've discovered that if I expect something, I'm unlikely to be upset or particulary uncomfortable with it. I expressed this to my partner and he agreed and said that for as much as possible he would always talk to me about any possibilities with other people (no matter how small) so that if something did come of it, I wouldn't be shocked.
2) Love scares me. When I first discovered that I loved my partner and he loved me, I freaked out. Love has a lot of power over people and can make each party very dependent. So the idea of them loving eachother scared me. It meant that not only were me and my partner vulnerable to eachother's feelings but he would also be vulnerable to his feelings for her and because of that I would be vulnerable to her feelings for him.
This is obviously a risk you take in any relationship but I have some deep seated issues with dependency, so I guess that's why it bothered me.
However now that I know that's what it was I can work on removing those issues.
Sorry for the belated and long-winded reply.