I sometimes wonder if shopping for groceries is as frustrating for most people. I like Nutella, but when I see Palm Oil as the second ingredient I picture dead orangutans. I like beef, but I sympathize for a fellow mammal and think of the enormous amounts of diverted water and fertilizer runoff that goes in to producing feed. I enjoy fish, but thinking of this makes me sick to my stomach. I enjoy many beverages that…
You can work yourself into knots over it; there is no way in which you can eat food and not have some negative impact. So, IMO, you either give up the battle entirely, or you can stop killing yourself over it but still make intelligent choices. For example, I eat fish because I believe fish farming could be a better choice for the future than cattle farming, and I eat chicken because the feed-to-meat ratio is better than beef.
As not only animals but meat-eating animals, it's just a fact of life that our/your existence costs something, means some sort of sacrifice for another organisim. You can try to reduce your impact, but you can't not have an impact.
Also, a little bit of my personal philosophy here- one way we could reduce our impact is by doing as little as possible, never moving about and burning more calories than absolutely necessary. Never performing strenuous activity that results in regenerative & anabolic processes, which require extra nutrients. But the way I see it, just existing costs the world "100 externality". If living to the fullest costs "110 externality", I feel like I am doing a disservice if I chose the first option, saving so little "externality" and spending my life miserable. For have I not then utterly wasted the costs I imposed on the world?