You can understand your partner’s attachment style if you dig into their childhood and have them open up to you. Or you can observe how they are around their parents or how they act in your relationship now.
When you understand your significant other’s attachment style, you get a real sense of where their insecurities are coming from. You can either talk to them about this directly or guide them to get help, AKA therapy. If they don’t get help and their attachment style keeps making you anxious, you may need to leave the relationship for your sanity and your partner’s.