Make sure you can look after your own basic needs first, so you're not requiring resources from others. Invest in your own education. Try to be there for others that you care about.
Realise that sometimes conflict is inevitable and you can't always be "good" to everyone. If there's conflict, resolve it quickly and decisively. Look for win-win solutions but be prepared to accept win-lose.
Look at the big picture - not eating meat, not having children, not using public transit doesn't necessarily help if others take your place. Do what you think is good, but if you believe in it, make the case to others. Real changes come from large-scale social movements.
The world is complex, and there might be more important things you can do for it than changing your diet or daily habits. Bringing up well-loved children can be a positive for the world.
Realise that everyone has difficulties. You can be the person who helps someone face their difficulties. You can't fix all the world's problems, but perhaps helping one person out might be more meaningful to you then chipping away at some of the big problems.
Be aware that there's no absolute good or bad. There's always people on both sides. Sometimes you just have to make a call. Like your point on using water in veg agriculture, there are plenty of people arguing for meat consumption, many of whom are invested in related industries. Being an evangelist for either side will likely not win you too many friends, but listening to arguments on both sides might help you come to a good compromise that can inspire others.
Follow your gut sometimes even when the data is not all there. Take time to think if you can. But make decisions and act. Aim to trust yourself, and if you don't, work on being someone you trust to make the right call when it matters.