It's so hard when your newborn/baby experience was a rough one with colic/reflux, etc. But, every baby really is different. Everyone I know who had a "difficult" first baby was granted a nice, quiet, easy one the second time around :-)
My two are 22 months apart and while it is always hard the first few weeks/months after a baby (whether it's your first or 5th, sleep-deprivation is always hard to live with!), it really gets better. My son and daughter play, play, play together and really enjoy each other a lot.
I have two friends who just had babies 18 months apart and from their experiences it seems like once you figure out how to handle sleep-deprivation and coordinate napping, they seem to have had an easier time than I did with mine nearly two years apart. I'm not sure what the difference is other than the fact that the older child is still a few months away from the "terrible two's" and in that adorable "honeymoon" stage of toddlerhood (really, 18-month-olds are darn cute most of the time, and even their fits are laughable!). By the time the older kiddo hits two, you'll have had a full night's sleep and be ready for battle :-) At that point, you'll be grateful for your unassuming, immobile little infant and trying to find ways to auction the older one off on ebay....
Hang in there!
It seems like you really adore your daughter, and are an attentive mom. Believe it or not, it is possible for those feelings to be multiplied to include another child. And the fact that your husband is excited about it is truly a bonus.