My personal actions make NO difference to overall climate effects. However, the trick is to make everyone believe the contrary so it DOES make a difference. That said, my view is that industry and cars are the biggest culprits.
My city put together stats and ad campaigns not too long ago.
The top source, 52% of the city's air pollution comes from tailpipe emissions. While mass transit and alternate forms of transportation are longer term solutions are good, the simplest recommendation is to buy a newer vehicle. The average vehicle is just over ten years old, meaning there are many vehicles from the 1990's and early 2000's out there. In general, newer vehicles get better fuel efficiency and have reduced emissions.
They estimated that if everyone replaced their vehicle with a new one and made no other changes, tailpipe emissions would drop by one third.
Heavy industry was the second highest air pollution source, at 18%.
Household sources were the third highest source of air pollution. Furnaces, water heaters, grills, and other sources of air pollution.
You are right that you are one individual among 7 billion other people. So you individually are a small amount, on the order of 0.000000X% of the pollution, more or less.
Individually your contribution is relatively small. One person making a few small changes won't do much.
Collectively, all seven billion people making a few small changes would be enormous.