What a lot of conflicting feelings to sort out OP - no wonder you're exhausted by it all.
You've had terrible experiences with marriage, and understandably aren't keen on another.
But you love your partner, who does want marriage, and a big wedding to boot.
So you're going along with his wishes.
But you both have very different tastes and expectations. So you don't want to leave the wedding organisation to him, and end up having a chaotic celebration that would make you feel miserable and resentful.
So you're having to get deeply involved in organising an event you don't want.
But your partner is making even this difficult, because he thinks you're being unnecessarily fussy and creating a huge workload for yourself. He's more easy-going and wants the kind of wedding that 'just happens'.
All this on top of a DD who needs extra care, a pending exam, employment worries, and no family in the country to turn to.
No wonder you're feeling lonely, overburdened and at your wits' end.
What's missing from this picture is an honest conversation between the two of you. It sounds like you've shared more on here than you ever have with him.
You say it would make him very sad to know how much you don't want to get married. But this is a huge part of you - the person he loves - who's been through trauma he can't imagine. Well, you're going to have to help him imagine it, if you want a long-term relationship with him. He sounds a good and decent man. But you're treating him like a child who can't bear too much reality.
Is that the problem - do you think too much reality might scare him off? Or are you making assumptions about him, maybe in a self-sabotaging way?
The fact that he's criticising your handling of the wedding, to me that shows just how far apart you are emotionally. If he knew what all this is taking out of you - as a loving partner he'd be much more supportive, and want to find a solution that works for both of you.
Whether you do or don't eventually get married, you can't keep on carrying these burdens alone. Please talk. It'll be painful but you'll both be better for it.