I'm sorry Viv!
Your worries are exactly why I'm worried about adding a second w/ Jackson... because he absolutely loves other dogs and lives them with them 2-3x a week when we stay at my dads house, etc, and I like to think he'd want a sibling, but I'm so scared of him giving me the 'cold shoulder' or not being happy, LOL!! But like everyone else said... give it time!
I brought Jackson home at 9 weeks old and my moms dog was 3yrs old at the time. When I brought Jackson in for the first time, he came in, sniffed around a little, and then curled up into a ball on the couch. Carmen, my moms dog, completely ignored him. She acted like he wasn't even in the room! It was so funny, I actually expected her to maybe bark more at him, or start acting jealous, but she acted like NOTHING had changed and went about her ways. I'd say it was about 3 weeks before she got the hang of him, and then decided it was okay to play with him... they turned into BEST buds and unfortunately, Carmen was hit by a car when Jackson was around 9 months old. He was really depressed about it for a while but then he got over it after a few weeks, so it goes to show you how fast a dog can change its mind about things. I KNOW everything will work out and that in a month, you will be posting stories and pictures of the two of them just being best buds!