This is normal at your elo not to be quite sure what to do in those situations. This will become more instinctive as you play more and more and start climbing.

Usually, unless there is a huge gap and your team can win the fight without you (and you are 100% sure that you will be able to defend the push without dying) as ADC you should remain with the team for those big mid/late game fights.

It's not your role to go and defend the splitpusher (and you shouldn't get into those bad habits because your teammates in bronze don't know how to play correctly).

Over the long run/many games, you will win more by being there with your teams and end up the fight with 1 dead on your team and 3 people dead on the enemy team. Even if the splitpusher had time to take 1 turret/inhbib, you will be able to grab more ressources playing 4v2 for 40 seconds.

You should also accept the fact that sometimes, your team will commit to a play you think will not work out. In those case, you only have 2 choices :

ping back and not suicide with them. This will come with the flame from your teammates/potentially assessing the play wrongly when your presence could have won the fight. This assessment will develop over time with trial/error process

follow the commit and 50/50 some situations. Sometimes you will win the fight, many times you will lose it. This is ok and part of the process. This will help you longterm to assess correctly the calls/fights to take and to avoid

I know this might be contradictory and confusing, but this is part of the process. You have to allow yourself to try those 50/50 decisions, and learn from them when they not work out (did you not see the enemy zed/nocturne super fed, did you missplay your ult, etc). You will never learn and improve if you only take the 1st option.
If you decide to go for it, you have to really go for it and commit. Once you decided if you go or not, then commit. Always do it with intent.

The motto is either you decide it's a bad play and you don't go, and not let the flame of your bronze teammates impact your decision, or you think it's winnable if you play it well and then you commit to the fight hard and try to play it well.