Hi
Need some moral guidance please 🙏🏻. I have been with my bf for a while now, months not years. Things are great, soulmate stuff and I never thought I’d meet someone that I’m so in love with. He feels the same. We both spent over 15 yrs married, both have kids under age of 12, they’ve all met and we spend time together and it’s great. His ex cheated on him, is with that guy, hasn’t introduced him to her kids, very secretive and she’s very jealous as a person so loathes our relationship. Bad news is she has stage three maybe four cancer. It’s terrible news, my bf (her ex) has taken it pretty bad, I can tell he’s deeply shocked. They haven’t told the kids but she’s just told them now she has bf and they’ve taken that very badly - I think it’s poss the way it happened as the youngest overheard her on the phone.
I’m finding this situ very hard. I sound selfish but I’m scared, I’m worried, I have had a very bad few years after the loss of my brother and have ptsd. I reacted weirdly when I heard and have said I will support him and her as much as I can but I’m finding it impossible to get my head around. Bf went and stayed at hers for dinner with kids the night he found out but the kids still don’t know because she doesn’t want them to know. They’re coming to mine for a break with him and she’s flipped. I don’t know what to do, I feel so sad for her, I care about his children, love them tbh, but I have also got this nagging feeling of fear and total guilt. Guilt of how happy I felt until this point and guilty to say - (please don’t judge) that I really can’t imagine going through another awful cancer time. I know, I should not be thinking about me but I have only just after years of clinical depression got myself straight and new bf helped that. 
I want to do the right thing, be supportive and help but I’m also pretty wobbly myself and run a business ft with 2 children a rather torrid ex myself. Anyone been through similar can you help me do the right things please?