Justification for staying: If you're sad because you're lonely, how will removing an intimate relationship from your life help that in any way?? Seems totally counter productive. In what way would it make you happier? It would be like if you had a close friend who you couldn't see very often, and you wanted a best friend who could hang with you every day, so you stopped being friends with the close friend. Just sort of a sad non sequitor.
Justification for leaving: If you think that having a secondary partner is making it harder for you to find a primary, then it would make sense to go.
Me, 30ish bi female, been doing solo poly for roughly 5 years. Gia, Clay, and Pike, my partners. Davis, ex/friend/"it's complicated." Eric, Gia's husband. Bee, Gia and Eric's toddler.