Whoever told you to stop nursing was an idiot. Breastmilk babies do get a bit more jaundiced than regular babies, but it's not that big a deal. You treat them like any other jaundiced newborn. Putting them on formula doesn't really help. It's too late now. I'm sure your milk has dried up. But that was bad advice. For other mothers in this situation, there are lots of breastfeeding resources out there to consult. Surprisingly, doctors are not very well informed on breastfeeding issues and their knee-jerk reaction when anything is going wrong with the baby is to try formula instead. In most cases this is the wrong solution and it's not one you can take back once your milk has dried up.
As for weight loss, your body is still recovering from the pregnancy. It's not ready to lose weight yet. Your hormones are such that they're not letting you lose much yet. You probably won't lose much more until you start cycling again, which actually shouldn't take too long since you're not nursing anymore.
Plus, it's only 6 been weeks. You shouldn't have even been exercising yet. Doctors generally tell you to wait until 6 weeks before returning to your exercise routine. Anyway, it takes time. Don't expect to bounce back to your normal size like a supermodel after her pregnancy. You've had 4 kids. It gets more difficult to lose the weight each time and the older you get the harder it is.
Advice: Just keep doing what you're doing and be patient. Don't expect to lose it quickly. It will probably take you a year or more to lose it all. I'm at just past 2 years and I still have 10 pounds of baby weight left to lose. I may never get back to my pre-baby size, though I haven't given up yet.
And, come on, being 20 pounds overweight is not the end of the world. Yes, you should try to lose it, but don't beat yourself up about it if you can't. Losing weight is hard.