Am really sorry to hear that you have been reduced to tears over this.
Short answer - your baby ,your choice, every one else should butt out.
It's awful that you are coming under so much pressure, especially from DH.
I suspect they do not understand or appreciate the benefits and mechanics of breast feeding.
Once you start giving formula, it will affect your supply, although some people do manage to continue with mixed feeding for some time.
When you feel ready, ( not when its convenient for MIL) you could drop one feed a day and replace it with a bottle.
However, you might want to think twice about giving the baby formula - it's cows milk, and some babies are allergic to it or at least don't tolerate it well. If there is a history of allergies, eg asthma or eczema is the family it is probably best avoided until the baby is at least a year. You could use soya milk, which is easily available, instead.
But best to express if you can. I found the easiest way to do this - ie no effort at all required on my part - was to use what i described as a drip tray. You get them from Mothercare/Boots, they are plastic shells with a hole for nipple to go in, put it inside your bra while feeding on the other side. The slight pressure causes you to leak - milk is collected, and can be frozen. The more you do it, the more you produce. Magic. Easy for you, easy for baby, as the taste is the same as always.
If you want to give a bottle, get someone else to do it until baby accepts it happily. And stay out of sight and sound of baby! Nothing worse than hearing your baby cry for what you could give easily. Let DH take the strain while you go out.
Good luck x