Breast milk and toddlers...

Sorry if this is an inappropriate question here, but I don't think i'm a member of the Breastfeeding community anymore....

So i'm pregnant, 30 weeks, and will be having baby in 9 weeks (induced). I plan to try and breastfeed, even though it didn't go well with DD #1. I tried and tried, and my milk never properly came in, and then when it did I tried (and failed) to relactate. Finally, a local IBCLC and my Dr found out that it's because of my meds that effects my milk supply. Not sure what i'm going to do about that, because I HAVE to be on these meds, but that's another post LOL If I succeed, and manage to have a decent supply this time around, I would like to exclusively breastfeed until 5 or so months. I feel really guilty that I didn't give DD #1 her best start and was only able to breastfeed the first 3 months, so I was wondering if it would be weird (or even beneficial at this point) to give her breastmilk after DD #2 is born? When DD #2 is born, DD #1 will be 21 months. Has anyone ever heard of this, or is it silly?