If you can't keep your back straight its probably because you are trying to lift too much weight. Although you think you can lift that much and you probably can, you need to work up your strength with lower weights as to not damage your back!
Go down to some lower weights on the Dead and progress along with your lower back.
Dead lift is probably the #1 injury prone exercise out there due to people not keeping their backs straight and trying to go
EGO vs Form!
Take it slow bro! it will come, it will come