They say: "you can love two people at the same time, but not at the same degree." This situation eliminates the "other half" and "soulmate" notions. A relationship is already as difficult as it can be, so what happens if instead of one, you have two relationships at the same time? How to handle this conflict?
If you're already in a relationship and you started developing a crush over someone else, slow down! Don't rush things and ruin what could have been your true love. Commit yourself to the person you are with.
You're thinking about this other person. You're creating and imagining scenarios and situations that don't even exist and wouldn't probably. Spend some time with your current partner. You might be overlooking some things or taking others for granted . Spending time will help you develop an idea of why you had a crush over someone else.
Instead of thinking that the new one to arrive is better than your current partner, take time to evaluate. Draw a list comparing the two: pros and cons. Relationships aren't always hearts and flowers, some reasoning is necessary.