You'll get 2 types of advice on SI.
Some people tell you to take specific actions to D or R. Some will claim you'll experience certain consequences from one course of action or another. Other people seem to want to help you figure out what's best for you.
My advice is to rely more on the advice aimed at helping you figure out what is best for you.
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Bigger's advice - to tell your W that you want to stay M, but that means dumping the ap - is excellent. If you're not ready to act on it, in your own way (I used fewer words, for example, but my W dumped her ap within 75 minutes of revealing her A), look inside and figure out what is keeping you from acting.
If fear is keeping you from acting, I'd recommend doing the 'simplified 180'. If you can't do that, my reco is to find a good IC. If you're waiting from something from your W, figure out what that is and how long you're willing to wait.
The people who come out of infidelity most successfully are the ones who take responsibility for their own lives. You may be wrong about what you need in order to act or what you're looking for from your WS, but if you monitor yourself and adjust to what you think is really happening in your life - as opposed to what you hope or fear is happening - you'll come out OK.
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Think about your requirements for R. MY reco is to include NC - no messages of any kind, no approaching the POSOM in any way, reporting any attempt by POSOM to break NC to you and deciding together how to respond.
Once you have your list of requirements, my reco is to talk with your W. If she agrees, great - R starts. If she won't agree, great - D starts.
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Keep asking for what you want. If the ratio of yes to no is high enough, great - R continues. If the ratio is low enough to keep you unhappy, great - R stops, and D starts.
If you can't ask, my reco is to find a good IC. If you ask and get too many 'noes' but you can't start D, my reco is the simplified 180.
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You're 3 months out. You've got plenty of reasons for acting now. You may have reasons for delay now that you haven't shared, which is OK. But take control of your life. Make the decisions you think are best, but know why you're making your choices, and don't lie to yourself.
You can survive and thrive after infidelity, if you live your life mindfully.
Where you are now - wanting R but knowing your W looks like a lousy candidate for R may be the worst your life will get. It'll stay at this low point until you know which way to move, and you probably need more time to know that.
But have faith in yourself to heal. Have faith in yourself to see the best course of action for you. Have faith in yourself to have the courage and strength you need to make the hard choices.
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I committed to R 90 days after d-day, because I wanted R, and my W looked like a great candidate for R. But I still had to go through months of uncertainty. At 2 years, I had mostly healed. Around 42-48 months out I was reconciled - but I didn't realize that for at least another year .
Others had much different timelines. Some took 5 or more years to declare success in R or to find victory in D.
So you know enough to start D now, but none of us knows the future. If you're not ready, so be it. Be true to yourself, but be true mindfully.