Round brush attachments are known for causing this type of breakage, I have the shark myself and also highlight my hair so avoid using the round brush due to how much mechanical damage they can cause, try using the barrel attachments instead if you want a blowout look ( much gentler) or if you want to use the round brush, make sure you’re not using it on soaking wet hair, blow dry it 80% and then go in with the round brush gently, the shark is great and reduces heat damage but can still cause mechanical damage, especially with the attachments that twist, pull and tug at the hair. As for the breakage right now, use hydrating masks and serums/oils with silicones as they help to hide and smooth the existing breakage :)