I would like to tell you about an idea that kind of worked for me. I’m sure you have some memories in your mind that creates the ‘soulmate’ effect. I always try to see those nice memories of people, whether friend or an ex-boyfriend, as my treat. The reason you see that ‘soulmate’ effect, is you actually. You were the one that was able to enjoy, mindfully observe the moment and luckily you still have those memories within you. This is never about the other person, you might still spend amazing time with someone who is not able to feel anything. People seem to mimic their feelings sometimes, even if he was not mimicking, you were the one creating that amazing effect. So maybe trying to see your effort and contribute to those memories maybe would help you getting detached from him. Those were your memories, you will have them forever and you’ll live amazing moments with yourself and others in the future as well. Good luck.