If you want to ask that, think about how many potential lives you've destroyed by not donating time and money to them. Just about any material goods you've purchased have promoted sketchy business practices with impoverished people. In many areas, donating food further destroys the livelihood of the areas' farmers ability to financially survive. There's the possibility the food will only allow them to continue their meager, sufferable existance for another few months. If they are barely surviving and you promote them being in a position where they have children, there will be more mouths to feed later, making the problem worse. By advancing medical technology, people will live longer and overpopulation will come sooner, or would be stifled at the price of more people suffering.
If the surgeon saves the lives of an American soldier, and the average K/D ratio for the enemy is, say, 6:1, then you are saving an American life at the price of six other ones. But if you let that American die, the war will drag on that much longer, adding to the death toll.
In terms of environmentalism, it you may be better off for the environment if you simply didn't exist, unless you have a net output of resources higher than you take in, but then you are only specializing in which environmental disaster you are allieviating.
Although your contributions may be minute, think about how even drinking a Coke is vaguely supporting the oppression of Brazilian worker's rights and unions, and switching to another soft drink would only make those problems change location.
There are no easy, one-dimensional choices when it comes to these things. If you donate to giving people higher education, you vaguely also help to oppress those who either didn't have that option or chose not to. Now, are you really going to burden yourself with the repercussions of your actions?
Edit: Please correct me if I'm wrong, I'll wholeheartedly listen.