Where's my motivation? It's a silly line from many a bad comedy, but when it comes to relationships, there is something to be said for honestly asking that question. When your sweetie brings someone into their life that you're not thrilled with, or you'd just rather not be sharing just then, where's your motivation?
Theoretically, we should (don't you just LOVE that word?) all be happy to share our partners at any given time with anyone that they are interested in.
Is your sweetie interested in enjoying things that you just don't like doing with this person? Does this date leave you with an extra slot on your social calendar that you would enjoy filling? Are they sweetness and light when they get home, and you get to benefit from all the endorphins floating around in the air? Does their sweetie have conversations with them that foster personal development you get to enjoy? Is there a new skill they bring to the table? Is your love extra attentive and appreciative of you after seeing their sweetie?
When you can answer the "What's in it for me?" question with something meaningful, this makes it so much more rewarding to block that time off on the calendar and enjoy the time apart. As the partner spending the time elsewhere, try to find some value for the person you aren't currently with. For example, PG is more of a night person than I am. We've had an on-going conversation for years about my desire for him to go to bed with me, and his desire to stay up and enjoy the evening. With the split in sleeping schedules, it's easier for me to stay up a little later, and for him to go to bed a bit earlier on the evenings we sleep together. The rest of the time, I can crash earlier in the evening with S, who's an early to bed, early to rise type. On the flip side, S gets up too early for me most of the time, but he can usually stay in a little later on mornings I share with him, but still enjoy the butt-crack of dawn on days where I'm happily snuggled up with my later-riser, PG. Being able to split sleeping arrangements also gives me someplace and someone to be with when either of them is entertaining other partners, which is a much happier space for me than solo time on the couch. ;)
Find your motivation. Be able to provide your partner with motivation. Enjoy the fruits of said motivation! Life and love are sweet.