Breastfeeding during pregnancy

Im almost 5 months PP & I’m 6.4 weeks pregnant . I can already notice my milk supply dropping and I’m devastated. I honest would have never gotten pregnant again if I knew I would lose my milk so fast i feel like I’ve failed my daughter, I can’t even breast feed her for a full year. When my milk comes back after the baby I plan on tandem feeding them both. Thankfully I have a sister in law who is an over supplier and her daughter won’t take a bottle so I still get to feed her BM just not mine 😔 has anyone been through something similar? Is there much I can do for my milk?