I feel guilty that this is another unplanned pregnancy that only took one go to fall pregnant and there are so many other women who struggle to have a baby and it seems to happen so easily for me despite my health issues.
Never feel guilty for being able to fall pregnant easily. You being pregnant doesn't take a baby away from anyone else.

Also, I personally love small gaps between babies. Gosh, minutes after my son was born I was already thinking about how much I wanted to give him a little sibling, lol. And if my husband would let me have another baby (which he won't ) I would want to be pregnant again as soon as I could. Having another baby won't make your DD any less of a baby herself. She'll just get the added experience of a baby brother or sister to interact with as well.

My son was 20 months when my daughter was born and they are already becoming close. At the moment my son loves to "Shayla kiss" and he gives her big cuddles and smooches. He also loves making her hold his little toys, hehe. It's a delight to watch and both you, your daughter, and your new baby will love it ♥