In my experience the best way to cope with past-life flashbacks is to realize what past, future and present really are.
He only exists in a cloud of probability. If you consciously or unconsciously choose to think about anything related to him, he exists. For you, in that moment. When you are thinking about something else or choose to be fully present in now, nothing else but the present moment exists.
If you want to be fully present with your boyfriend, you should cut all the ways for your ex to reach out to you. Don't let him know your phone number. Don't let him know your address. Minimize the possible interactions. Take the responsibility of your mental health and don't let anyone from your past to disturb you. If you want to let go of his unnecessary torture, you simply need to accept what happened between you and him.
I don't know what stage of coping with PTSD are you in, but it seems like you still need to process what happened in your life. Processing what happened is healthy and necessary. Practice self-love and self-acceptance.
I wish you the best on your healing journey.
