I think BoffinMum speaks absolute sense too.
I struggled with breastfeeding, with all three of mine - for various reasons. Ds2 ended up in hospital, labelled 'Failure to Thrive' on my milk - despite feeding all the time - and we weren't discharged until I started topping him up with formula, and he started gaining weight - and that spelled the end to breastfeeding.
With ds3, I mixed-fed, almost from the word go - and as long as he was getting at least two formula feeds in each 24 hours, he would gain weight at a reasonable rate. If I dropped it to one, his weight gain stopped dead. So I breastfed during the day, and then the last feed of the evening, and the middle of the night feed were formula - and that enabled me to carry on breastfeeding for 12 weeks - substantially longer than I managed with either ds1 or ds2.
Once I started weaning the boys, I worked hard to make sure I was feeding them as healthily as possible. With ds1, I was making and freezing all sorts of purees, in cubes, so he could have meals containing four or five different things, including different vegetables, carbs and protein. With ds2 and ds3, I didn't do that - I didn't have the time or energy - but I was cooking home made food, and blitzing that up for them.
There is so much you can do to help your child grow up healthy - and breastfeeding is a part of that, but it is not the single biggest factor, nor is it a really big factor, imo. It might help brain development, but so does reading with your child, playing with them, letting them learn a musical instrument, doing crafts. You can give a child a wonderful, healthy, balanced diet once they are weaned (though, in my experience, as soon as the little blighters get their own money, the amount of energy drinks and junk food consumed rockets - they still love good, healthy, home cooked food, but love a filthy kebab or cheap nuggets too - sigh).
I put myself under huge pressure about the whole breastfeeding thing - for ages I said I had failed at breastfeeding, and beat myself up about it terribly - and I am sure it contributed to my developing PND each time (though I do have a long history of depression - undiagnosed at that point - anyway). The mother's mental, emotional and physical health matter too, and I hope that the OP's husband appreciates this - I am worried that he doesn't, and that the pressure he is putting on the OP to exclusively express, will damage her physical, mental and emotional health.