What am I doing wrong that i can't backflip?

You aren't jumping high enough.

You're jumping back instead of up.

You aren't "setting" your arms (swinging them up strong).

You aren't pulling your knees up fast enough.

The whole thing is too weak and slow.

Stop trying to do this by yourself - you're going to get hurt. Find a gymnastics club and take a tumbling class, or a few private lessons. An experienced coach will teach you the correct technique and keep you from getting hurt.

You are chucking yourself backwards, instead of driving your hips and toes up and forwards, so it is just fundamentally completely wrong, for all that you think you "know the steps on paper." Also you never even straighten through your legs and hips in the initial jump. You don't show good body tension at any point: You have no set and your tuck is loose and floppy. You appear to have no "aerial sense" i.e. awareness of your position, orientation, and movement in space; or indeed any awareness and control of your own movement, as borne out by your inability to tell what you are doing wrong.

It is a total dead-loss and an accident waiting to happen. A back somersault is not something you can self-teach either safely or effectively, and even if that weren't the case or you had proper coaching and safety equipment you are clearly nowhere near being ready to learn one.

Actually you are doing very good, as a first step.

Next you need to get a competent spotter to train with you until you can get enough height to flip to the other side.