Some people can emotionally detatch while they're still married and even living together (either in an "open" marriage or by cheating, which is awful). Others need to have the divorce papers filed. Still others need to have the divorce final. I needed a few months after the divorce was final before I was ready. It's all very individual.
You may very well be one of those who can not feel available until the paperwork is filed.
Your stbx will not file as long as she is comfortable in her current situation. You need to file. YOU are not comfortable in the current situation. Your only reason for not filing is some hope that it MEANS something that the paperwork is not filed. You KNOW that to HER it means nothing, but to YOU it DOES.
And because it does, you need to file. It's not that hard, just turn your ability to focus on a problem (and we KNOW that you, of all people, can focus on something) onto the issue of finding the paperwork & filling it out, and get it started. You'll probably find that just doing that is not a magic bullet that tkaes care of everything, but at least it'll be FILED and the process will have been started for you to heal.
Until you do, the process for your healign from the rejection will not even start.
At some point, I will be saying to you that a lunch flirtation is NOT a commitment, it's NOT an important thing, it's JUST the opportunity for talk, flirtation, maybe a hug or kiss. But for now, because you are one of those who feels still attached without the paperwokr being filed, even the THOUGHT of a flirtation, hug or (gasp) kiss is offensive.
Get to a point where it is not offensive... file... THEN we'll talk about how much fun it is to reconnect with old friends or have lunch with a pretty girl.