Milehighquad might have the right idea about deepening your relationship with this guy, it's worth a try, sometimes it works but it is very dependent on the parties. I would have liked to have a great relationship with my metamour and we do have a good relationship but there is nothing in it that makes sharing my partner with her any easier.
Maybe you should find a girlfriend of your own? I've been doing this for three years now as a mono in a relationship with a poly guy and it hasn't really gotten any easier consistently. There have been times when I've been very positive about it but then the struggle always seems to come back.