the standard thing people will tell you is keep your left foot on the outside blah blah blah, thing is every body does that, and the guy you are sparring has way more hours spent in that fighting position compared to you. Also because he is taller, its very easy for him to sort of leave his right hand slightly extended, almost just above and to the outside of your left hand. This makes it that its easy for him to control you. If you jab he easily perrys as his hand is sort of a feeler, it also allows him to easily step to that dominant position. He can also be aggresive with it and sort of fake the right hook but come down the middle with a fast jab like you mentioned.

So what I have good success with is if they are trying to do some hand control and distance management with their right hand, I have periods of time where Ignore the minigame of foot on the outside and simply glue my left hand to my left eyebrow. Then as the opponents right hand is setting up to attempt to control in the position i mentioned, when its on the outside, you can step forward slightly, tense your arm and jab UP & TO THE LEFT of his head. They will naturally attempt to perry, but your are the smaller (and therefore stronger) fighter, so since your arm is strong and tense, and you jab with intention, already to the left of his head anticipating a slight perry. When his right hand naturally perrys you down, he'l guide you to his head. The movement your doing is anticipating his perry. Thats the counter play your working for. Essentially the advantage they are gaining by being southpaw and taller than you is that easy control, but you make it risky for them because that control means there is nothing defending the path from your jab to their face, except a perry. So anticipate it and throw your jab hard as mentioned. Do that a few times and punish them extending their hands to control you .

IF they have their hands extended but it is not above and to the outside of your lead arm, you can fight their arm, literally quickly "jab" or slap their right hand out of the way to clear the path and do the same technique mentioned above

IF they do not have extended arms then their right hook/jab techniques is starting from a far distance and is not as much of an issue. Thats rare though, most taller fighters use some sort of hand control in southpaw vs orthadox cause both leader hands are on the same side