I feel for you BexAubs20. Big hug. You're not going crazy. Sadly GPs nowadays lack 'bedside manners' and it sounds like you're totally burnt out. This happened to me 5 years ago, lots of similar symptoms. It is fixable and you must put yourself first for a change.
Sadly NHS tests are not broad ranging so it's like finding a needle in a haystack. Get a 2nd or 3rd GP opinion and keep pushing or go private if you can.
I forced myself to practice mindfulness daily, a walk with a podcast on, breathing exercises, everyone can manage half an hour a day. This helps enormously because our brains and guts are directly linked and stress has devastating effects on the body. The pain in your joints will only get worse if you stop moving, I used to have to stretch in bed before getting up. Sleep is massively important for our wellbeing and health. After surviving on around 6hrs a night for over 20yrs, I now know I need to have 9hrs and used to go to bed same time as my kids when they were little.
It's good you're taking some supplements but I can also highly recommend:
Counselling - get everything off your chest and professional 'tools' to cope,
Having a good cry, scream into a pillow, anything to release that tension,
A 'Mummy MOT', superb for pelvic floor/post natal issues,
Eat lots of oily fish, eggs, nuts and leafy greens - get your 5 a day religiously (joints)
Up your dose of Vitamin D, this affects A LOT of things,
Vitamin C the body cannot store, strawberries, apples, oranges etc x3/day
Try a proper iron supplement like Ferroglobin, or a liquid one by Floradix,
Floradix liquid Magnesium (gut and joints)
Solar Ultimate Calm,
Folic Acid - not just for pregnant women
Just check and make sure to not over do the same vitamins, especially B vits. Most vitamins in excess the body expels.
Chin up. Hope you start to feel better soon x