Encountering former, abusive advisor at a conference
This doesn’t answer your explicit question, but have you considered sending an email to the conference organizers asking for your poster to be reassigned to a different part of the poster presentation area so that it’s not right next to the former adviser’s poster? This could go a long way towards minimizing the possibility of trouble, and will probably make it easier for you to handle the task you have set yourself of overcoming the anxiety you have related to meeting him at the conference, while leaving you some wiggle room to maneuver in case that turns out to be more difficult than you expected.
The email doesn’t have to go into details, just say this is a person you’ve had some conflict with in the past and you would prefer for your poster not to be close to theirs. If I were an organizer I’d be more than happy to accommodate such a request.
Good luck, I hope things go smoothly and that you’re able to focus on sharing your work with others and having fun.
Be natural
Greet him the first time you meet him, shake his hand, say "how's family," or whatever suits the circumstances. For you, what happened is water under the bridge. You've parted ways and you're now under a different advisor. He's got no power over your career. Even if he tries to subtly insult you (which I think he won't, because as you said, he's simply ignoring you since), there is absolutely no reason for you to feel bad about it. If he indeed does so, that would be pathetic of him and you should feel pity about him. Worry about the important stuff and the people that matter to you; not for grown-ups whose behavior is stuck in 6th grade and you've only known for a year or less.
Do NOT ignore him!
Ignoring him will only bring more awkwardness to the situation and more stress to you. Imagine the set-up. You're both going to be in the same place for quite some time (a couple of hours?) and in close proximity; you'll both be standing in front of your poster and there's going to be times where there won't be anyone around asking questions. Actually, there might be moments where he's going to be the only one around. Trying to ignore him will only add more pressure to you ("oh my god, we've just had eye contact! aaaargh!").
Ask for help
Having been through similar situations with my PhD advisor in the past, I agree with Noah's comment that you need to get help to overcome your panic attacks. It doesn't necessarily need to be a therapist; it can be friends, family, trustworthy people you feel comfortable discussing it with. You need to learn to manage such situations because you'll get many of these in your life. To move forward, you need to stand on your feet and handle them, not ignoring them and praying they won't happen, because they will, anyway.
Ignoring him and refusing to respond to any uncomfortable advances or comments might be best.
You don't owe him anything, especially the satisfaction of making you feel uncomfortable. Focus on your poster and on interacting with those who are interested in it. Do what you should do anyway and make some professional contacts at the session.
There is no need to do anything more than if the next position over was by someone unknown to you and whose work you aren't interested in.
You have just as much right to your space as anyone else. Dominate that space and try to let the rest go. With practice such uncomfortable encounters, that may occur from time to time with others, will become easier to manage.
And by "dominate that space" my intention is beyond the confines of a poster location.
There is an outside chance that the conference leadership would respond positively to a request to be moved. "I'd be more comfortable elsewhere" is all you need for an answer if asked why.