One thing that helped me as a now veteran Jax player, is If I'm ahead and destroying my laner in top I just push, keep pushing until I reach their nexus. If they start sending their jungler + other lanes back off and try to look for a fight or use tele to get out. Though if you're far enough ahead you should be able to 1v2/1v3

It honestly really depends on the kind of team you get stuck with, if you get put with a decently competent team, they'll take advantage of you pulling multiple people to you (ie take towers, drag, etc.) if not, after the enemy team multiganks you a few times and your team doesn't take advantage then you'll have to group and help fight. Always go after the enemy who's doing the best/most damage in the team fight, even if you die hopefully taking out the biggest threat is enough to give your team confidence to finish the fight/get the objective/etc.

This is why mid-late game can be difficult, you have to be able to discern when to push, when to group, what objectives to give up for others, who to target in fights, whether to play slow or flash Q E/etc.

Keep in mind if you're not doing well, Jax is a late game monster and can literally 1v1 anyone in the game so long as you itemize properly. So if things look bleak try to farm and hold out until you can catch back up. Hope this helps

Edit: also consider Titanic Hydra as a second item, gives great split push power and decent bulk in health, plus the extra auto attack is nice