Mama, you are doing the right thing by nursing your DD when she needs it. Listen to your baby not your doctor. In my experience, pediatricians are good with medical issues but are very old school when it comes to parenting.
I have an 18 month old son who initially needed rocking or nursing to sleep. When he was about 11 months, I found I could lay him down on the bed next to me, sing to him, and he would drift off to sleep - so that's how he goes to sleep at night now and for nap time. And I take a nap right along side him! We always go to him when he wakes up and cries, and give him a cuddle or boobie to get back to sleep. I want him to know that I'm always here for him, whenever he needs me.
Your DD is only 6 months and that is still so young. She needs to know that mama is close by and that you are there for her. By soothing and comforting her at nighttime, you are teaching her that sleep is a safe and pleasant state to be in.
I do not believe in "sleep training" because all it really teaches bubs is that if they cry, no one is there for them. What a cruel lesson for them to learn.
And I think the concept of "self soothing" is a crock....again....I think that "self soothing" just means that bubs has given up crying because they believe they are all alone, so what is the point.
You are doing a great job mama.
Remember, just because someone else offers parenting advice, does not mean you need to take it.