Never tolerate abuse. You deserve better. Remember to be polite to your partners, they deserve it even more than the stranger down the street. Try not to save all your best stuff for one partner and take your partners for granted, especially when they are together. Try to treat them evenly or someone will feel slighted.
Comparisons make people compete and force someone to be the loser.
Find a way for everyone to win. Respect relationships as well as people. Think of each relationship as a separate entity that could be healthy or sick. If you need therapy, see a doctor. It's easy to project your expectations onto other people.