OP, some of what you say rigs warning bells, but other parts resonate with me because I felt desperately that I wanted a second child in the long-term, but also struggled terribly with a hard first child and my own physical health. He was hard because he had silent reflux which stopped him sleeping and speech problems for the first 3 years of life, and although we did sign language it was pretty hard going until he was better and could finally talk. By this time we were on our knees and scared to have another. I have multiple sclerosis so get desperately tired with the neuro-fatigue, as well as battle with physical symptoms that make daily life extremely hard. On paper it made no sense to have a second but emotionally I felt I was grieving the child I would never have.
I would ask other people how they coped with two instead of one, and I remember one in-law telling me that "of course they had two, otherwise it would have been selfish". At first I was angry because I felt I wasn't being selfish, but more sensible given the circumstances.
Many people who grew up as only children said they'd been fine, but I still had a nagging doubt, and it was the only children who reported wishing for a sibling all their life that I felt fed something in me, in my decision-making process. I suppose you could say they were telling me what I wanted to hear. The final crunch was a woman with MS who had grown up with her Mum seriously ill with MS too, and she said she had been really grateful for a sibling to share the worry over their sick Mum. That was it for me!
I fell pregnant with my second and lo and behold had another child with silent reflux and sleeplessness. Physically it was really hard again, and I relapsed again too. My husband took me to relate because he was unhappy and we nearly didn't make it. But the second child is growing and becoming easier, and I kept myself going with the knowledge that one day they will both be in school, then work, then they will be adults that are part of my family and not just very dependent children sapping all my strength.
They fight and I have to steer them into learning to share and to use words before fists, but they also love each other fiercely and those moments are precious. I am so glad we had a sibling for the first, the type of personality he is would have meant he pined for another person in the house. In our circumstances it is better because there is less pressure to always be out on play-dates with school friends, something I struggled with doing too much of physically, due to MS.
If you and your husband parted in the future even without a second child, would you wish you had two anyway or would you be weak with relief that you stopped at one? People have two children and split up then meet someone else, some people stay single when they split up. There's no knowing what your future will be. I would dare to suggest you should decide what's right for you irrespective of a man, because men come and go but your decisions don't . That said, I'm sure you'll survive having two and you'll make a success of having one as well, so what I'm saying is, either way isn't the end of the world. Sometimes life just happens and doesn't always turn out how we expect.
Also, part of choosing to do what's right for you means getting your health sorted. You won't have a comfortable pregnancy if you're carrying all that extra weight, and whether you have one or two children, you'll be better off finding a way to control your eating. I wish you luck with that.
Certainly having any new addition to the family brings a huge amount of strain on any relationship, even a good one, but it sounds from what you say that yours already has quite a few cracks in it already so it would be easy to think you have an increased risk.
Sorry if any of my honesty has hurt, that is not my intention at all. But there's no other way to be helpful other than to say it as it is. 