I need to lose 80 pounds... help?

I switched my antidepressants about two months ago, and I've gained 15 pounds since then. Now I'm tipping the scales at 240 pounds and six feet tall...

I work at a desk for 40 hours a week. I've never lost weight with exercise (except for when I rode 100 miles a week on a racing bicycle a few years ago, but I was hit by a pickup truck and haven't touched a bike in 7 years), so I need to change my diet again. I used to weigh 158 pounds when I ate nothing but unprocessed meat and eggs, but that is too expensive now. I need to eliminate sugar and carbs from my diet, but I don't know how to do it without wrecking my health like I did last time (I developed permanent hypothyroidism). Help?
I need to lose 80 pounds... help?
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