If you are going to continue to practice with aluminum, you really should get some something other than that narrow flat stock.
It looks like you are building the heat up in the material until it puddles, but since the piece is so thin and narrow, it can't dissipate and it eventually washes out and burns through. This is precisely why pulsing, or using a pulser, comes in so handy close to edges or on thinner material. It stops from building up excess heat in your workpiece. Unfortunately for you, though, all your welds are basically edge (or close to edge) welds, because of how narrow that flat stock is.
Stick with it! You'll get there.
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