Woah, someone who writes longer posts than I do...at last!!
There's a lot, a massive lot of stuff going on in your head - it's not surprising you feel confused.
Many of the things you mention are indeed common to, well if not all of us then a great majority during a pregnancy. There tends to be a balance of positives and negatives and I think definitely a sense of loss of control, and a fear about how things will change, is usually present.
Yes, babies do change the plans but you can't always predict whether that is good or bad...and in what ways. For instance the bedroom thing, you might opt to have the new child in with you initially, and ds might then decide he wants a larger space in time for you to put them both in together...something like that. Things don't always happen as you think they will.
Wanting the 'old you' back is understandable and again very very common - I totally relate to the physical stuff, I too used to be thin and wore nice clothes and after pregnancy, cried about the loss of that - but there is a common thread with some of your fears, and that is that perhaps a second pregnancy would actually improve certain things...you might end up slightly altered but it might be in a good way. Especially the psychological aspects, to do with your changed role etc...I'm much happier as a mother of two than I ever was as a mother of one - not sure why, I just am! It's bizarre and I don't know if we're different on that, as you say you dote on ds already, and I never did find things easy with my first. Likewise a good friend of mine finds her second child a challenge, while she enjoyed her first very much. But still she is glad she has both.
The question of what would be best for ds is a good one. I always wanted to give ds1 a sibling, mainly because I am rubbish socially and don't have many friends, hate playdates and so on...so I figured that a sibling would be a really good idea, and actually it has been, despite a large age gap (4 years). They really get on and they appreciate each other deeply, in a way I honestl;y never anticipated...and it actually takes a LOT of pressure off me as they have someone present, someone to muck about with and talk to, an ally against big cross Mummy at times as well
There is another relationship now - not just me and ds1, or me and ds2, but ds1 and ds2. And fwiw I think that is the most important one in our family.
Obviously there's a chance you will resent your second child, or your ds might not get on with him/her, or you'll feel worse. But I would think that in your current mindset the element of uncertainty suggests a bias towards regret.
I spoke to several counsellors when I was trying to decide, and none of them said much that I found helpful, as it wasn't about them, but me and the other people involved (my now ex, who wanted me to have an abortion, my parents and friends, and ds1)
but they DI say, and this sticks out, that if you are as little as 10% unsure, that probably means you'll find it very hard to get over or at least experience considerable regret. They said, unless you KNOW in your heart, taking everything else away, that you DON'T want this child, then it will hurt you to do this and there is a risk you'll feel worse afterwards than you did before. Saying that, there are probably a lot of women who don't feel that bad and are mightily relieved afterwards!
I think when I had taken everything else out of the equation (I had HG - severe m/s) and little to no support, a child already, was single etc etc) in my heart I didn't want to abort my baby. To me it felt like drowning an unwanted kitten - the sentimental element was too strong for me to ignore, but then I am the type to pick up a hurt pigeon and take it to the vets where it usually dies anyway so thatw as partly why I felt so stupid. It wasn't a head decision for me - it was a heart one.
If in your heart you know this is right, and you'll be happier if you lay this little one to rest, then that's probably what you ought to do.
I believe you need to follow your OWN deepest feelings. And I also don't think your partner is likely to leave you over it, whatever you do. He sounds like a good bloke.
Will be thinking of you, anyway.