Hi Blanch. Welcome to Psych Central. I am sorry you feel hurt as your ex has explored other romantic situations.

I think this sounds like a bipolar behavior in the manic phase. It does not mean your relationship was not valid, but the person you were in that relationship, may have had untreated bipolar mania which can lead to all kinds of risky behavior and can destroy relationships.

This article offers some insights into bipolar relationships. What Happens to Love in a Bipolar Relationship

Rather than blame yourself or discredit your relationship, you may want to research more about bipolar hypomania and see that it was not your ex did not love you, but they were disabled by the bipolar and may have even had psychotic breaks. They may have even saddled their new relationship with all the baggage that led to your break up with your ex. So now that person is struggling like you were. Sad as that is, maybe in one sense it could be a relief you no longer have to carry the burdens that bipolar places on a relationship.