I sorta get what Boyla is saying there. Practice the jab like a throw without the intention of a big bang on it. (there is a big difference between a straight hard shot with intent and a straight jab).
Once you got it so you can repeat the jab continually you will find that you can stop someone dead in their tracks with it by having your body movment stepping in behind it or timing them coming into it with a slight adjustment to avoid theirs but make yours move in further.

Confirm; has it down with the angles thing,footwork/ timing and the power will come later.

The elbow down vertical fist jab is best used if you outreach the opponent or are taller and you move slightly and shoot your jab through and over the top of and from the outside of their extended jab; blocking their shot and simultaniously having them walk into yours. Your feet can take your head slightly to the outside of their jab while you adjust stance on the angle so you outreach them and counter them in one shot.

Kostya loved to fight southies because that arm he kept out as a feeler intead of the jabber ; got in their way and difussed or got in the way of their jabs just by him turning slightly and staying behind it ,then he would lean away from their other hand throwing that right hand bomb when they didnt move away.

You could also (from the inside)try dropping your glove into the elbow point of their jab and turning into it and walk through and out, pushing them open while moving away from their other hand throwing your left as you crouch in and turn. (your left hand landing will be in the way of them throwing their right ,more so if your leaning or moving that other way.)

Physically; the right hander is at exactly the same disposition as you are when fighting you.

Thought about another way, You actually have an advantage; that the righties are more used to fighting each other ,more often than people like you!