I don't feel like I can give you too much advice since I have only been here for a couple of months myself. I can offer you support and my own experiences so far (I have some ways to go). Your sitch is similar to mine in that my H just was 'done!', possible EA as well, suddenly can't remember any good times, everything is my fault etc. I went into a tailspin too, did all the begging and pleading - and it DID NOT HELP one bit. It also made me personally feel worse.
So, I totally agree with sandi2, step away. Give them what they THINK AND SAY they want = freedom and space. At first it will feel totally unnatural to you. And, it is. But hold on to your hat as best you can, and the feelings will begin to soften. The S eventually does realise it's not really what they want, but they need to see it for themselves. Nothing you can say to them will get through to them as they have this ongoing argument going on in their own heads.
Giving space is probably the hardest thing because you worry if it will make him even more distanced and less feeling. It doesn't - trust me. It is just a fear you have.
After giving him space, I managed to focus on some of my 180's - which is actually VERY hard work, and sometimes leaves me confused. I am grateful to all the people here who post and share their experiences as there is a lot of hard-won wisdom (backed up by experience). So read here and it will calm you down, and give you pointers.
Just to clarify - 180's are things you wouldn't normally do. So, if you are a yeller (like I am), instead keep quiet. Find alternative modes of communication - and see it as you expanding your way of being. It has not been easy for me as I am like a volcano, so I've learned to use distraction, and this forum instead and other methods to express myself (like through my art). I have no idea if it is working on my H, but it certainly has worked in other departments of my life, hence my other relationships are improving and I feel more empowered.
So, good luck and keep posting I appreciate that I can be supportive to someone else.