I used to be poor. The worst parts were the nights when I couldn't find a friend to stay with and I slept in a park, or in the woods, or a burned out building, both because the nighttime in Colorado is pretty cold, and because it's just kind of scary to not know if you will get jumped, or have your (meager) possessions stolen, etc.
It also kind of sucked not to have any real form of transportation, and to have to depend on others to get anywhere. I mostly got rides with friends, but sometimes would get stuck in one town or another at an inconvenient time, which would of course often lead to a night in the open.
The other part that was really unpleasant was if I didn't get a good chance to do laundry for a few weeks. Even if you spend a night at a friend's house and take a shower, it's very unpleasant to get out of the shower, dry off, and put on 2-week-old clothes. It just feels gross, and of course it's very embarrassing because you know it's unpleasant for those around you as well.
Food wasn't so bad. In 1996 I could get a box of Pop Tarts for $2.19, and that came to about 1700 calories. So I could get by with that and some multivitamins. I did get down to about 150lbs (I'm 6'2" and from a family of big farmers and carpenters, I'm normally about 250) but that was not due just to food and happened over about 2 years.
You didn't specifically ask about this, but I'd like to give credit where it's due for the two things that helped me break out of some bad mental, physical, and social cycles
Help from family, which was invaluable in getting me the first couple steps back on track. Especially my aunt who lived in Los Angeles and let me move out with her from Colorado when I was quite a risk
Attending the San Diego Job Corps, where I lived from the end of 97 through August 2000. I earned my GED, learned electronics as a trade, attended community college, and was accepted to UC Santa Cruz while I was there. Absolutely wonderful program that completely changed my life for the better.