Hairdressers shudder when hearing the terms "chemical cut, toasted, cotton wool or chewing gum" when used in relation to hair condition. Repeated lightening, overlapping of colour, over processing, too high volume of developer, heated appliances, tying hair tightly and/or in the same position or a combination of these can result in breakage on already fragile hair.
What to do?
Low & slow - lower volume developer over a longer period.
Use Snag free hair ties & loosely tie hair.
Precisely place colour to avoid overlap.
Time colour & chemical services & check frequently. Do not use heat. Use even amounts of protein & moisture products.
Finally, regular trims help immensely getting hair back from the brink!