too much weight lost- milk supply down....

So my chunk munk right here is a good eater! We breastfeed mostly and only supplement when needed here and there. She's 4 months and already 19 pounds... I am down past my pre pregnancy weight heading to an unhealthy low weight and I noticed my supply drop dramatically these past few weeks... which made me extremely upset ( i want to breastfeed for a year or more) it's like she's sucking everything out of me! I'm sluggish, depressed sometimes, and my skin gets more loose the more weight I lose.. (down 45 pounds- gained 30 in pregnancy) 
Has anyone else struggled with this? Were you able to still breastfeed? Should I continue to breastfeed? 😩 idk...