Many people feel that if you claim to be having an exclusive relationship and do not follow through on that claim, that that is immoral. To have strong feelings and a romantic interest in more than one person is common and I suppose rational, although what we seem to enjoy most in relationships aren't rational but are intuitive and even magical. One has to decide for oneself in conjunction with your partner where to go and what decisions to make when you have feelings for more than one person. What happens differs in every case. Some people get out and regret it.
Some people get out and feel their lives are greatly improved.
You start over again by mustering up your courage, making decisions on your own, and going step by step into a new life with confidence and courage. Some people find therapists very helpful in this situation. He always lingers on situations forever with his parents and overall general problems. I love him tremendously, but sometimes this gets out of hand. Letting the past go is often an important issue in therapy.