Let us assume that such a situation will apply to women, although the mechanisms (that I am going to talk about) zwork for men as well. Magda has been in a long term relationship. She loves her partner Adam, has two children with him and feels happy with him.
One day, however, a new guy, Darek, joins her team at work. At first Magda treats him like a usual colleague and sometimes even tells Adam about him, but after a few months of working together, they started having feelings for each other. Their relationship turns into an affair and Magda falls in love with her colleague. Does it mean that suddenly all feelings for Adam have disappeared for good? No. But at the same time, Darek's appearance in her office clearly affects her relationship with Adam.
In this respect, our feelings can be compared to pulling a rope. The closer Magda becomes to Darek, the further she will move away from her current partner. But the closer she moves to Adam, the more she will move away from Darek. Magda, like no other person, cannot consciously decide who she falls in love with, which is why she tosses around like a boat on waves. Sometimes it will seem to her that she knows which way she wants to go, but then it can change unexpectedly. Depending on what happens in both her relationships and the behaviour of both men, her involvement will change accordingly. Let's see how it looks in practice.
Magda doesn't want to leave Adam, because she still has some feeling for him and she has children with him. Separation would be associated with big changes in her life and pressure from her environment, so she prefers to avoid it and have her affair on the side. She found herself torn between two relationships - her feelings for Adam after so many years are already in a rather advanced and mature phase.
For a long time there has been no excitement, and their relationship is based primarily on friendship - and now it has become rather dimmed because of her romance. But feelings are nothing more than chemistry in our brains, so the neural connections responsible for love for Adam remain somewhat dormant. The new relationship with Darek is in the phase of early love. It's about him that Magda can't stop thinking and can't wait to meet him. It is also the phase of idealizing the other person and attributing the best qualities to him/her. In such a situation Darek may seem much more attractive than Adam, even if in other circumstances she wouldn't normally think so.
When everything seems to favour Darek, who was previously a single and is totally in love with Magda, suddenly something changes. Adam clearly feels that something is wrong in his relationship and sees that Magda is cheating on him, which he doesn't like very much. But instead of trying to control Magda, proving her unfaithfulness or even asking her not to leave him, he takes a different attitude.
One day he tells her: "Magda, you denied it many times, but I know that someone has appeared in your life. It's not easy for me, but I know that if you fell in love with someone, it didn't depend on you. That's why I don't blame you, I don't hold any grudges against you and I'll always want the best for you. At the same time, I don't want such a relationship to continue and we have to break up. I hope you understand me and you will forgive me and we will still be able to get along and raise our children together.
Darek was initially delighted with how the whole situation turned out, but quickly he saw that something was wrong. Magda suddenly distanced herself and wasn't as warm as before. When her partner broke up with her, she felt a big loss and started to miss him. At one point she realized that if she did not take any steps, she could lose Adam forever. Adam, breaking up with her, pulled the rope to his side and at the same time Magda's involvement in the relationship with Darek definitely lowered.
At that moment Darek, seeing that he was losing Magda, started to fight for her at all costs, persuading her to be with him and trying to contact her constantly, which pushed her even further away from him and pushed her into Adam's arms.
This is how the exemplary dynamics of feelings look when someone loves two people at the same time. Sometimes the action is even more dynamic and the course of events changes many times in favour of one side or the other.