From reading your OP and update, I can't imagine why you have any doubts at all? It all sounds perfect.
But to answer the question of, "how do you know"....well, you just don't. I was with first H for 20 years (married 17 yrs) and I could never have known when I married him, that he would be a habitual cheat. Fucking soul destroying! He was my first BF, and I was so shocked 16 years in, to find out that he'd cheated the whole time. When I finally left him 4 years later, he beat me up twice and tried to stiff me financially. I didn't see any of it coming.
But you have a choice in life. You either let something like that swear you off men for life, or you get back up and decide that there are decent men out there and you deserve true love. But of course, it makes you sensitive.
I have been with my 2nd DH now for 8.5 years, and we celebrate our 4th wedding anniversary tomorrow (yey!). I honestly think we will be together for life.
Most of my friends are in 2nd marriages. I actually think there's a tendency to marry too young, say in your 20's, when you haven't played the field, you don't really know what you want, but marriage and babies is expected, and maybe as your near late 20's/early 30's you feel you had better get on with it as the biological clock is ticking.
In my experience (mine and all of my friends) the men that married in their 20's didn't adapt well as the marriage evolved. In almost all cases, they had kids, and the wife is suddenly mid 30's, sleep deprived, not up for much sex, pre-occupied with the babies (which is totally understandable), but the guy didn't take the journey with her, he carried on going out and getting pissed, and then eventually fucking about, because his wife got "boring". Well, Doh, she's looking after her "D"H, running a house, maybe working too and raising kids.
Anyway, I think when you get together in your 40's, all of that "grunt" work is over. You don't have to find someone to procreate (you've both had your kids), so you get together, purely to just be together. You have also done the rounds/sowed your oats, and choose freely to be with that one person.
All of the 2nd marriages I know, have worked.
Having said all of the above, you can never know if one person will change/fall out of love and fuck off. So please be sure to make pre-nup arrangements and wills that ensure you are financially protected!