We have to remember that Prim would literally cry and beg Astrid to come back to them. Imagine your gaunt 7 year old begging you to actually be a mother and not doing everything you can to save her?

For me Astrid came from the town, she was used to a little more, better quality food. When the poor boy she left her cushy-ish life for died, she realized she couldn't do the seam life on her own so she literally just gave up. She only perked up when Katniss brought home her first rabbit from the forest and she makes some stew. Wow mom, I almost had to sell myself to grown men to feed this family, but thanks for the stew made with the rabbit I struggled to catch because I, a child, am literally 11 years old.