Thanks to all who've replied with suggestions and shared their experiences. I've read many of the "supposed to" and "should" replies with trepidation however as they don't necessarily apply to myself or my husband. I'm very reassured that there are people out there with a similar situation to ourselves.
To clarify some of the questions/points that have been brought up. I definitely spend more time with my husband, however my boyfriend lives downstairs so I do see him every day as well - even if only for a short time.
My husband and I have always had a sex life that seemed to "ebb and flow" even before we introduced other partners into our lives.
I'm coming to understand that perhaps we are not as sexually compatible as we are with our other partners. We've grown and discovered things about ourselves sexually and been able to find very fulfilling things with our other partners and I think that makes both of us feel very guilty. In all other ways, our relationships works and we do love each other, the life we have together makes us happy. Accepting that things are different and learning to appreciate where we are now, then I think we can look ahead to how we want our relationship to work.
Thanks again to all who shared