I feel guilty for getting pregnant...

I have a 5 year old and an 18 month old daughter. I'm 15 weeks pregnant with baby #3. I haven't enjoyed this pregnancy at all and yes, it was planned. 
I keep finding myself almost wishing that I hadn't gotten pregnant and that it was just the two girls again. I feel guilty and feel as though I'm taking so much from my other kids by having this baby. 
I hate feeling this way. It's sad and scary honestly. 
I feel so lost :( 
My husband just keeps saying it'll get better...but what if it doesn't? I've been a shitty mom since getting pregnant...moody and spent weeks with my head in the toilet or sleeping. I just feel so much guilt and I don't know what to do. 
I just need support