attributes reseted after death or not applying

When i add an attribute to me (as a player) e.g. generic.max_health i can set it how much i want it but when i die it shows me the extra hearts but they cannot heal and if i try to get my base after i have changed and i have died it is 20 (minecraft base before i apply the change). This happens with armor and attack speed as well

Secondarily if i try to change my speed attribute it only applies the changes briefly (1 tick) and then just resets my speed to the original state.