So sorry to hear about this horrible predicament you are now in. As a survivor of domestic violence who used to be terrified of running into my ex, I deeply sympathise with you in this situation. From your brief description of the relationship with your ex, it sounds like he was emotionally abusive to you at the very least, and the behaviour you describe indicates he could possibly be a narcissist.
You are absolutely NOT being unreasonable to feel this way, ESPECIALLY as you were still involved with this man as recently as 3 months ago. Even though you now can clearly see him as the toxic person he is, 3 months would be far too soon to expect yourself to have fully got over the pain he put you through.
Unfortunately, practically, there’s nothing you can do to avoid running into him as he lives so close by, and I’m assuming you wouldn’t want to move now that you are settled in your new home. And, if your ex IS a narcissist, then I strongly suspect he deliberately moved somewhere to be close to you in order to cause you further distress and to rub his new relationship in your face!
If you can’t move away, the only thing you can do is seek healing from the relationship with your ex, through seeking therapy if you can afford it, and/or reading books about healing from narcissistic abuse.
Dr. Ramani has written a great book called It’s Not You, and she has also made lots of free YouTube videos AND runs an online support community for people who are being or have been abused by narcissistic people, and I thoroughly recommend her.
If you have supportive healthy friendships, also focus on those.
And when you inevitably run into your ex, do your best to pretend you haven’t noticed him or appear indifferent. People like him crave getting any reaction from their victims. You can break down in tears you are safely home, but try your best not to let him see he is triggering you. ❤️
It’s a horrendous and difficult process, but it will slowly get easier with time and healing.