Whose voice carries more weight when campaigning to reduce fossil fuel usage: the person who profits off fossil fuels and will suffer as a direct result, or the person who has divested themselves completely?

If you have a moral objection to receiving those funds, then just give them to a charity that might actually help.

The way economics works, if you choose to divest yourself in this way, you are betting against the profitable use of fossil fuels, but you are not influencing it (it will either continue or cease on its own depending on whether it stays profitable). You are simply encouraging someone else (who maybe doesn't care so much about the environment) to "take that bet" and profit off your poor decision.