Right now, he is in control of your life - but only because you have ceded control to him. And it's time to take it back.
He's got you doing the "Pick Me" dance. Telling you he might leave or he might not, "his mind keeps changing" and all the while he expects you to passively sit on the sidelines waiting for The Master to finally decide. You're still doing the cooking, the cleaning, his laundry, possibly sharing your bed with him, being all wifely, while he dithers over whether he is still a husband. Enough.
Tell him that you are tired of his ever-changing mind, and thus YOU have made up your mind - he is to leave. He is to pack a bag and move out to a hotel or whatever, you have decided that he cannot possibly make a decision whilst he remains in the family home, moving out will give him space to think clearly. He is not to contact you for three days, and then you will have a conversation about the future. This is what you SAY to him, and you SAY it in a way that he believes you mean it.
The situation will then go one of two ways. One, he panics. He realises you are not doing the Pick Me dance and he is no longer in control of the situation, he might genuinely lose his comfortable life. Or, two - he thinks he can shag the other woman now and skips off happily. (How happy she'll be is another matter - I'd say it's 50:50 at best that she'll entertain the idea.) Either way, you bring this shitshow to a head, and possibly stop it in it's tracks.
You say you are prepared for the worst, by which I presume you mean the end of your marriage. So set the ball rolling - you're prepared for the worst already, and it might not happen if you take control now. If you allow it to drag on, your marriage may never recover even if he stays.
Stop dancing. Take control. Tell him to leave.