I really don't think this is necessarily anything like hot wife or cuckoldry stuff. And I've NEVER heard any female post anywhere that they were having hot threesomes with their husband and new girlfriend and the other two people seem more into it so that they were somehow getting off on humiliation, or inferior just because the other girl seemed more talented or desirable in bed. That said, what redpepper said in the other thread about maybe it's time to move away if you don't feel the threesomes are benefiting you as much as much anymore would probably work.
It also doesn't sound like new lover means better lover. New often has more energy and excitement, just like you and she used to have. Just like if you take a lover you'll probably have for awhile. That you happen to be the old comfy nesting partner is likely more a byproduct of cohabitation and long term ease. It's likely some of the energy does come from you being there and supporting the dynamic. I imagine it's nice to be that comfortable with your partner that you can share that without feeling bad about your desires for somebody else. Never had a threesome, but in a MFM scenario, that would probably be a good 33% of what would appeal to me, knowing that my other partner was supportive and glad to see me have so much pleasure, that it was a team effort, and comfortable for everybody.
I also haven't found that bigger means better at all. Skill, love, desire, passion, attention, and awareness of how your partner is feeling are sure a lot more important than penis size to me. Well..and knowing when more lube is needed before I have to say anything
You say you'd derive no pleasure from being inferior - are you feeling this at all or are you just noodling around the topic in your head or putting out out for conversation for fun? Feeling inferior because your partner has NRE (sexual or emotional) or feeling inferior because you think somebody else is better in bed are two totally different things.