Addendum: Don't tell your wife you're getting an attorney. Just get one. Don't express any interest in the two of you "sharing" a lawyer. (I doubt she would.)

You sound like a decent guy. I'm sorry you are in this very hurtful situation. It's got to be one of life's worst pains.

Life dealt your wife a very tough hand with the birth of your step-son. She was rather young when it happened, too. It doesn't get easier. I'll bet her life must feel pretty "boring" a lot of the time. On top of that, all marriages get boring. It would be understandable, if she got angry and depressed. Life is demanding that she mature fast. She's not sounding real mature to me. I think she is recklessly going down a road that isn't going to turn out real well for her. She feels cheated and may have decided that "nice guys finish last." So she's dumping her nice, boring husband, figuring she better go-for-the-gusto, while she's still quite young. This is short-sighted. How many guys are going to be up for partnering with a woman who has a special needs child?
( She was dang lucky to have found you.) It sounds like she wants to run wild for awhile with no thought of where that will land her.

Women who want out of a marriage sometimes play this game of being separated, but not actually pursuing divorce. That can be a way of milking you to the max. I don't know your wife. You must have seen some good in her. She probably has her virtues. But don't go along with being made into a complete sap. Get counsel. Trying to pacify her with romantic gestures is not working, and it likely won't. She'll just see you as more "boring." Make yourself a little mysterious. Keep up your relationship with the kids, but don't ask a lot of questions about what she's thinking and planning. You'll only get B.S. answers about how she's "working on herself." That's bunk. I think you may be excessively tolerant. Women don't want martyrs. They prefer warriors. Toughen up. When you are in her presence, mostly listen. Speak minimally. She'll be quizzing you to see where your head is at. Leave her wondering.

You are probably the more mature spouse. If you want to fight for this marriage (which might make you less boring,) see the lawyer. Then, unless legal counsel objects with good reason, get back in that house, even if you sleep on the couch. Your stance toward your wife could be, "Either put up, or shut up. If you want me out of here, do what you have to do." Then just go about daily life. Don't plead and cajole. She has already made up her mind whether or not she intends to divorce you. Let her put the cards on the table and stop playing with your head, which is what she's doing. Nevermind her taking responsibility for the marital rift. She never will. But, if you can get life back to some kind of normal cohabitation, even if it's sexless, there may be a shot at derailing the divorce. Stop dancing to the tune she plays. She does not know where she's going in life.