Your routine is what is destroying both your skin and your hair - 40 vol peroxide is highly alkaline so horrible for both, cut back to maximum 20 volume and aim to only hit the roots not the lengths because repeat dying is one of the worst things you can do. All you need is an applicator brush, two mirrors, good lighting and some patience to do your own roots professionally. The lazy route is an applicator bottle with the tip snipped off, squirt a 'zig zag' of product into each part line and massage into your root area with gloved fingers - there is a little more overlap but it is faster.

If you are using a weaker persulphate bleach/ peroxide mix you have way more time to work slowly and methodically, you can even bleach hair in two sections/ two sessions so you have more flexibility with time. When you touch up your lengths there is no need to hit it with peroxide/ persulphate bleach, use a high pigment semi permanent colour like La Riche Directions or Manic Panic which is completely non damaging. With reds this actually lasts better than a permanent red as long as you care for your hair well when washing, and 'fades' bright instead of dull.

Unfortunately nothing can repair hair that is completely fried because hair is dead, all you can do is cut or grow out the damage and stop abusing your hair. Overnight coconut oil soaks on dry hair can help reduce porosity and increase elasticity, also bleaching right over the top of a heavy coconut oiling protects from some of the damage. Hydrolysed protein treatments are also valuable for patch repairing and temporarily strengthening, and again may protect against some of the bleach damage.

Are you using a heat protectant? Useful articles on heat styling
THE NATURAL HAVEN: Straightening Facts :How hot is too hot?
P&G Beauty & Grooming | Bubble Hair
THE NATURAL HAVEN: Thermal protection: Do they really work?
How To Protect Your Hair From Heat Damage

If you have not already done so get your skin complaint diagnosed by your family doctor or a dermatologist, be sure you let them know you have been using 40 vol peroxide followed by 20 vol peroxide. Dandruff is often seborrhoeic dermatitis, but I suspect yours is partly or wholly down to you damaging the skin barrier/ protective acid mantle/ skin flora with your harsh routine (contact dermatitis). Do review your diet making it as anti inflammatory as possible and consider either a sulphate free shampoo at pH 4.5 to 5.5 or conditioner only washing depending on your diagnosis.

2a-2c, medium texture, porous/ colour treated. Three years CG. Past bra strap length heading for waist.

CO-wash: Inecto coconut/ Elvive Volume Collagen
Treatments: Komaza Care Matani, coconut/ sweet almond/ fractionated coconut oils, Hairveda Sitrinillah
Leave in: Fructis Sleek & Shine (old), Gliss Ultimate Volume, various Elvive
Styler: Umberto Giannini jelly, Au Naturale styling gelee
Flour sack towel, pixie diffuse or air dry.
Experimenting with: benign neglect