Scared, don't cut it

Abit of backstory, I have 2 kids (6 & 2) I have just found out that I'm 5 and a half we pregnant (going from last period). Scared don't even cut it, I've just settled into life, things are going so good, home life is thriving, everything is so so sound and bliss. Going from 1 to 2 kids was easy for me my second slotted right in, I found having just one a lot harder personally and I'm so so scared that I won't cope. It took me a long long while to become the mum and women I am today. It's meant to be happy moment and for some reason my head and my mind is ruining it, can we afford another child, will I be able to give all 3 the loving they deserve from me, will i be able to stay afloat. I guess I'm just looking for some reassurance that 3 isn't so bad, I'm scared, I feel awful for even having second thoughts, I'm going to see my gp to talk about some of my worries not sure exactly how that may help but I feel so lost, not only in myself but in parenthood.