Really feeling depressed about a surprise 3rd baby please help...?

im 8 weeks pregnant with my 3rd baby i have a 3 year old and a just 1 year old. Me and my husband decided we werent having anymore he has been booked in for a vasectomy and i have given everything baby away i have no maternity clothes left either!! So it has come as a real shock. Hes really excited about and it and thinks it will be lovely to have another, so do all our family and friends we have told but im just horrified i cant seem to feel excited about it, all i feel i keep doing is feeling negative about all a new baby comes with. I feel like im getting depressed about it and really dont want to feel this way. Can anyone help me has anyone else been through this? How did you get excited about it? I have my first doc appointment next week and was hoping they can send me for a dating scan and i can hear the heart beat hopefully that will make me feel a bit more excited about it but i feel horrible that im not excited its not like my children are in their teens and im going right back to square one but im just having a hard time with this.