You have four parathyroids and they control your calcium metabolism (and magnesium also). They don't influence your weight, the thyroid does that. You will currently have high blood calcium levels and this affects the brain causing distress. So, bear in mind some of the concern you have may be aggravated by this chemical affect on your brain. This will resolve after the operation.
I've not got a lot of knowledge on this but I don't believe it is nasty or painful surgery. The surgery is delicate, they make a horizontal incision in your neck, usually along a skin fold. Eventually the scar disappears. I've known one young lady who had her thyroid removed and another who had parathyroids removed. They never mentioned anything about the surgery being nasty. It may be sore for a few days but they will prescribe pain killers.
You may only need one parathyroid removed if only one is affected. If this is the case you may well not notice the loss. Usually, if they have to remove one or two parathyroids they take out the healthy ones as well and transfer them into your arm! This is not daft, it's so that if the others become diseased it's a very simple operation to remove them, no need to go near the neck again.
You will be referred to a surgeon. It's a really good idea to make a brief list of any questions you have and ask them when you see the surgeon. There is a UK parathyroid patient support group hypopara.org.uk/home.php , you could join them.