It's probably really overwhelming reading all these different ways to make up formula.
Some people saying do this, others saying do that, wait this long, wait that long etc etc etc. I don't see any need to boil the kettle and time how long before using the water. I just used it shortly after it finished boiling.
Anyway. Most people do make them up in batches enough for 1 day/night, flash cool and then store in the fridge. Then warm them up as and when they need them.
Everyone does it their way but as long as the feeds are made up with boiling water in the first place that is the important thing.
It really isn't a faff. It is made out to be for some reason. But it's not.
Compared to breastfeeding, of course it isn't as straight forward and there is more to think about but it's not this big faff that takes up loads of time.
We had 2 full sets of bottles, more than enough for 2 days of feeds. About 12-14.
One set being scrubbed/washed after use and put in the steriliser ready to be sterilised once all the bottles were in there. One set in the fridge with formula in.
Once you get used to it, it shouldn't take more than 5-10 minutes a day to make up a batch, cool, and put in the fridge.
Get a small cooler lunch bag with mini ice bricks, and then it's just a case of remembering to take as many feeds out with you as you need.
Tommee tippee do a travel warmer which is essentially a large flask to take boiling water out with you to warm bottles up.
Everything packed neatly in the basket of our pram, or in the changing bag.
In writing it sounds faffy, but it's fine.
Try to relax and stop making yourself feel bad, this is the way it has to be so she can gain weight, it's not your fault!