If you agreed on a monogamous relationship and you find out your lover messes around with others it’s to be expected that it arouses strong emotions, like anger, sadness or nagging doubts. Jealousy is a completely natural response.
Don’t blame your ‘rival’ and don’t compare yourself to him or her.
It is not your shortcomings that are at stake or the qualities of your rival. It’s about your lover deciding against monogamy, no longer being loyal to you. You should ask yourself what this tells you about your relationship. Were you going through a hard time in your relationship? Is it just a (nasty) way to signal something is wrong and should be fixed? Or is your lover someone who will not commit, even if your relationship is fine?
If you want a monogamous relationship and your lover is unwilling to commit, look for someone else.