Bleaching your hair comes with damage, but the level of damage varies for each person. I personally have found multiple ways to prevent and treat damage from bleaching (I bleach my hair at home)! First, the night before you are planning on bleaching your hair, soak your hair in coconut oil, like really soak it. Coat every strand with the oil and sleep on it overnight (preferably at least 12 hours). If you cannot sleep on it, try and let it soak for at least 2-4 hours before bleaching it. This will not affect the bleach at all, but protect your hair!
If you are bleaching your hair at home, I recommend getting the Ion Brilliance Absolute Perfection Booster (steps 1 and 2). Sally's Beauty stores sells it for around $2, which is an amazing price for an Olaplex dupe! I used both along with the coconut oil and my hair felt amazing, even healthy, after bleaching.
Before and after bleaching use hair masks and deep conditioning treatments. I do this year round, but the few weeks leading up to a bleaching I spend extra time conditionings and using masks to prepare my hair for the bleach.
Generally, just take good care of your hair before and after and your hair will thank you for it! The more effort you put into keeping your hair healthy, the more results you will see. Since your hair is already damaged, treatment takes more time than prevention, but it is worth it 100%. Just wait and take very good care of your hair (minimal heat, deep conditioning, etc) and you will see results