I don't know how to teach my toddler

She's 18 months and she's not hitting milestones. She's behind. I know it, my husband knows it, everyone knows it, but they're all giving me the same "they all learn at their own pace" line. It doesn't help me feel less like a failure.

She started off good. Could say mama, dada, baba (bottle), bye bye, and something else that I can't remember. She'd wave and nod/shake her head. She communicated pretty well.

All of that is gone. She no longer says words in relation to anything. She babbles still, like will say mamamama over and over or baba or whatever, but as far as I can tell, none of it means anything. She doesn't nod/shake her head no anymore. She has never pointed. She doesn't mimic anything we do unless it's something she's already doing. For example, if she's sticking her tongue out, I can get her to continue doing it over and over. But if I encourage it when she isn't already doing it, she won't do it.

She doesn't know a single body part/animal/color. I can't get her to pay attention to anything. I have board books, but she rips them out of my hand and only wants to open/close them. I have the electronic books that talk when you press the buttons, and she loves it. But she still doesn't mimic it. I put on Ms Rachel and she's only learned ball, but it comes out as bah. She doesn't pretend play at all. All she wants to do is run around, play with her ring stacker, and thumb through her books and fine motor thingies.

She isn't deaf or hard of hearing. I've whispered words to her before and she repeated what I was saying (wiggle wiggle is about the only thing she willingly says). She just isn't interested in anything I do. I don't know what to do anymore. How can I help teach her if she doesn't seem to care? What do I do? She's so behind and I feel awful!