Watching your loved one fall in love.
Nothing can really get you ready for watching your partner fall for someone else! It can really knock you on your ass, and i think it makes lots of sense that you're feeling put out. It's hard to see someone you love giving something you really want/cherish/desire to someone else when you're not really enjoying that in your own relationship. There's some great wisdom on her in that you can refocus on building a relationship that you really love with her in the time that you have together. do exciting things, confide in her, build that love and sexual desire fire up!!
Don't resent, it jes' makes it worse. It's here, it's not going away, so try to find your peace. Hugs - it's not easy.