Jesse was a loose cannon prone to melting down and doing something astoundingly self destructive and stupid at any moment. If he wants to destroy himself, fine. If I were in Walt’s shoes, there’s no way in hell I’d have let him destroy me with his incredible weakness.
His girlfriend WAS clean and sober when she met him. He’s the one who got her back on drugs which is what killed her. Jesse destroys everybody around him. With him out of the way, Walt had his operation locked down tight. He even walked away from it all and was in the clear. Then once again, Jesse happened.
Walter was his own personal worst enemy, not Jesse. Walter constantly made self-destructive choices as he descended into further and further madness with no way back to sanity.
He was decisive but then would change his mind about things he agreed to. How much was enough? Always just a little more. And that is the beauty of his character.
We discover quite deep in the series that part of his underlying motivation is resentment over being separated from a company, of which he was a principle member and helped to build, losing a potential fortune and all of the good life that goes with it, to become a high school teacher. He was far too smart for that. And then he gets cancer.
Every choice he makes is reasonable when seeing things through the lens of his character. But from outside it is possible to see the resentment and pride he has that ultimately become his undoing. His underlying commitment to doing the right thing was not based on principle but pragmatism, unlike his brother-in-law who was willing to die for what he believed in.
Walter was a stickler for doing things the right way, but only if the right way could clearly be seen to pay off in the here and now. His lack of faith in the afterlife and eternal rewards (and punishment) gave him no rational way to handle the injustice of getting cancer after doing all the right things. Rather than being willing to leave his wife and son with a legacy of faith and moral values, he felt he had to leave them money. In the end, the things he loved most—his wife, son, and unborn child—were damaged most by his choices.
Jesse was a deeply flawed person—an addict. He had burned bridges with his family, but we can see his disappointment in not being able to undo the harm he did in youthful foolishness. Yes, he fails to kick his addiction, even though he has tried. His girlfriend becomes a new motivation for him to get clean and sober, but he keeps getting pulled back into the drug life because of his relationship with Walter even when he is starting to head the right direction. It is true that IF he had kicked the drug life completely before meeting the girlfriend, she probably would have stayed clean too (and lived). He is the character of the whole series who cares most about other people. He is disappointed in himself for letting others down. This is why his character is the one who survives and gets a chance at redemption after the series ends.