HI, I'm not sure this is the right place to post this, but I'm in a really f*d up situation and could use some advice... (sorry for the long read, but there is a tl;dr at the end).

So I'll try and be as brief as possible. But it's complicated and thus I first have to provide a minimum back-story:

I've been separated from my wife for four+ years and have a son who is 3+ (it was an impulsive 'accident', i know it was a mistake, but that's another story). He lives with his mom. The reason we broke up was basically because she has (diagnosed) borderline personality disorder and it was impossible. Again I won't go into details but once she tried to stab me with a knife and her psychological manipulation was driving me to suicide. I started drinking a lot which just made everything worse... Finally, I left. The only reason we have not formally divorced is so that I can keep my son on health insurance.

Anyway, I've recovered and have also met an incredibly amazing woman that I'm in love with (about 1.5 years now, I have not told my Ex). I had already taken action to formalize the divorce (even paid a lawyer appx. 3k) when all-of-a-sudden I get a call from my Ex saying that 'she has something to tell me'. So I agree to meet and she tells me that 'she is sorry for everything'... her behavior, etc... much more was said but the details aren't important. I am certain she is sincere, and is crying and wants to get back together. I tell her that I cannot get back together. I thought 'that was that'.

Well, then a couple days ago we have a lawyer meeting to decide upon mediation and terms. We've had previous meetings and my Ex (and her lawyer) were very aggressive and basically threatened to take everything... but this time all my Ex said was that they have no terms and 'whatever I want to do'. This was incredibly confusing. Also her demeanor was very strange. After the meeting, she takes me aside and says, 'there's more' and proceeds to tell me that she has stage 1 breast cancer and will be having surgery in Jan. She tells me that this has caused her to rethink everything and doesn't want a divorce and she'll do anything I want if we can somehow get back together, mainly because she wants to have a family for our son, and she can't imagine being with anybody else. She also said that she understands that her disorder was the root cause of our problems and that she is now getting serious therapy. Again lots of crying and a desperate plea from her to at least try. At this point i'm speechless, and tell her I have to think about things. She asks if there is 'anyone else' and I proceed to lie and say no. That was a bad thing, but that's what I did. I'm not proud of it, but I simply couldn't process everything and tell her the truth in the goddamn lobby.

Now, to be clear I love my son very much, and he is awesome, an I cannot imagine life without him. To my Ex's credit she has done a great job of raising him so far (except he is a bit spoiled). My Ex is independently very wealthy but has no job and probably could be running out. I don't know. It's not really a major factor but just FYI. I do have major concerns that my Ex's disorder will eventually be conveyed to my son. This is a major concern for me. I have been told by our joint psychiatrist that my Ex is extremely manipulative and even dangerous. After a couple of (joint) meetings my psychiatrist literally told me that she cannot even see my Ex anymore because she is too dangerous (no kidding).

Now, with all that said, here's the impossible problem. I am madly in love with the girl that I'm with, but (and it's a huge but), I cannot fathom the idea of abandoning my Ex and son. I also cannot fathom the idea of abandoning my current SO who loves me very much, and will be completely devastated. But, I know in my heart that if I deny my Ex that she will take my son away. She has far more financial means than I do. My current job ends Jan 1st and right now I have few prospects.

So I have no good options. I will summarize:

1. Say nothing. proceed with divorce and try for some kind of joint custody. Without a job that's unlikely. Mentally find a way to deal with the fact that I've left my Ex when she has cancer. F*ck.
2. Tell my Ex about my SO and this is why I cannot 'come back', also given that I have little trust that she will reform, and I cannot go back to the psychological disaster that has already ruined my life. This will also ensure divorce and me losing my son. But, at least I was honest. But, F*ck.
3. Tell my SO that I cannot continue our relationship, basically betraying her and resign myself to a life that is beyond betrayal. Go back to my Ex in a limited fashion (I will not live there, I have my own life, for what little is left) but at least I will still have my son and as I grow older maybe it will work out. It's not like my Ex is going to die or anything, the prognosis is good because they caught it early. Still, I cannot see how to do this. F*ck.

The problem is that I cannot live with any of these options, and at this time I'm psychologically paralyzed.

Tl;Dr: It boils down to this: Divorce my Ex when she has cancer, and likely lose my son too, which I cannot accept. Or, leave my SO. She is The woman that I have always dreamed of. And it could destroy her in the process. Right now it seems like an impossible situation.

Perhaps some of you have other options to suggest.

I have no further words to describe the dilemma, the pain, and misery.

Any suggestions or insights would be very helpful.

Thanksinadvance.

Also, any suggestions for a sub-reddit are welcome.