Where to start with WAS

[Traveler]

I found out she was still seeing the AP 4 months into counseling,

I'm sorry your partner was unfaithful, and the type to slide onto another partner rather than face your marital issues. After so long apart, your odds are long. Many people here willing to help.

I don't know why she hasn't filed and had me served (I have theories),

The simplest explanation is, there's no benefit to her in doing so. I mean, this is why I and other WAS I know--with lousy ex's, of course ;)--waited years to finalize.

Should I just tell her to go ahead and file?

No, try not to tell others what to do, that's controlling. If you want a divorce, you can talk to your attorney and either a) file or b) sign and return the papers she sent you.

I think there are some people you just can't please and my wife is one of them. I sacrificed and compromised a lot for her in the marriage, but she could never reciprocate and was unwilling to fulfill my needs

I agree. Some people are not a good fit for each other. You say you didn't enjoy her company as she was--you wanted her to change and sacrifice to fulfill your needs. One of the more useful exercises a marriage counselor had me and my ex do was list down all the NEEDS we were asking our partners to fulfill. The MC's point was there shouldn't be so many! Have you now figured out how to satisfy most of your needs yourself, or are you struggling without a partner?

She has rewritten history and is moving on with her life, erasing me from her memory. I also don't want to give her any ammunition where she could put any blame on me for the divorce before God

You don't control her narrative. You only control your actions.

I think there are some people you just can't please and my wife is one of them. How would you approach the situation if you wanted to try one last time to save the marriage?

A starting point might be identifying and owning your mistakes. She owns her affair, but defining your wife as "unpleasable" puts the onus for the broken marriage that preceded it on her as well. By this I don't mean a hand-waving, "Blame is usually 50/50" or "I didn't value what I had". Where did you go wrong? What have you been consistent doing differently since? This sort of introspection and self-improvement helps get back together and helps move on.