There were a few things that really jumped out at me, the most notable being the you really push your punches, another being that your chin is right up in the air. And you were right; you do go straight back every time. Other than your chin in the air, I think all of this can be traced back to your feet.
The video is primarily waist up, but I'm pretty sure your feet are a bit wide apart at the start, and this is compounded when you step with your jab and leave your left foot behind. At that point you have made it impossible to throw a right hook, and you are reaching and pawing with your straight left. Other than the jab, evry punch involves a shift of weight: it begins on one foot and ends on the other. Now, if you are throwing a straight left at an opponent that is 18" away, and your feet are 36" apart, your hand will arrive long before your weight- a pushed or slapping blow.
Real basic, real dull series of exercises: get in your stance and get your feet under your shoulders (and, as a southpaw, being so squared up is really counter-productive;it takes away the evil advantage being a southpaw gives you). Taking small shuffling steps and move forward- right foot (maybe 2"), then bring the left foot up. Then go backwards- left foot, then right foot. Then mix in punches- every time your right foot moves, jab, then fire the straight left as you bring up your left foot. Doing going backwards too.
And right here you should learn something- an advantage to be gained going straight back. Throwing your left with these short steps you'll feel like there's nothing on it: DO NOT compensate by yaking longer steps. Compensate by really torquing your hips and shoulders (this is something I don't see you doing in your videos), and really turning over your right leg. I watched both of these videos all the way through and everybody comes straight forward after a guy going straight back; master this straight left going backwards, and you'll get some knockdowns.
Which isn't to say that you should keep making a habit of it, because you shouldn't. Minimize going to your left and start pivoting/circling to your right behind your jab: this labbing/circling to the right is why many people believe all southpaws should be drowned at birth. From your stance- remember, right shoulder frontward, feet under your shoulders- slide your right foot to the right just a couple inches, jabbing as you do so, then pivot on that right foot, until you are back to your original position and, as your left foot settles, straight left hand. (That is of course, to remind you to bring your left foot with you as you drill. In competition you don't need to throw the left every time, though I would advise bringing along your left foot as often as possible. But that constant jab and circle to the right will help ypou score points and keep you out of the way of punches. He CANNOT hit you if you are outside his left arm. Also as he turns to follow you, it will open up opportunities for a right hook and stright left.)
When you do move to your left, I would suggest it be for a specific purpose only; not it seems that you move that way habitually, and that you don't punch when moving left. That is because you move left foot first. Try this. Begin by sliding your right foot first, a small bit and jabbing as you do so, then pivot to your left (and throw a left). This is, of course, a drill and practice it, but, when you shadow box, I think you'll see how this can drastically and suddenly change the punching angles and open up avenuse for the straight left and right hook.
When you have these drills down moving short distances, hands and feet in coordination, start moving further and faster. It is what I call a 'fencing step'- James Toney and Ricardo Lopez were masters at it. Push off your back foot, sliding the front foot forward: two or three should get you across the ring, then go backwards, pushing off your front foot. Then work in your hands; going forward, jab as your front foot moves, so that your arm is fully extended as the foot settles. Going backwards, jab as you push off (can you see the value in this?). Work in your left hand, throwing it as you as you bring the left foot forward (when going forward), or, when going backwards, as the right foot comes back.
I'm sorry if this is long and confusing- probably should've broken it up in two or three pieces. Drill your feet and you'll notice a lot of improvement'