Congratulations! I can remember what my reaction was when I found out I was preggers with my 2nd. She was a surprise and I think the exact words were "holy s*** " I was happy to be having a 2nd, but I was also scared to death. My kids are 22 months apart, so I totally know what you're going through and what you will be going through. One of the hard parts is that #1 is still so fresh in your mind that you still have a clear memory of being up all night, all of the "bad" parts of pregnancy and all of the challenges of having a newborn. Then this time around you throw in a toddler that you have to take care of and it is very hard. But you will do it and it will all be worth it.
I did feel kinda bad because I wasn't 100% for my son when pregnant. There were days that I was so tired that he would play on the floor next to the couch while I was laying down. But even if you can't be there 100% physically for your son, you will still love him just as much and that's what is most important to both of you. Plus, as your pregnancy goes on he will be growing up and become more independant, that will help. Nothing will change the bond you have with your 1st. If anything, having a second baby makes the 1st bond even stronger. Because you will not take for granted all of the special alone times that you have together. And when the baby comes you will set aside special times for just the two of you. And it will be really cool.
In my experience, having 2 close together was quite a challenge in the beginning. My pregnancy was no fun. Then going from 1 kid to 2 wasn't like doubling the work, it felt like triple the work. But as the months went by it got easier and easier. My youngest is now 15 months and I am having so much fun with my two kids! They are best buddies and have just reached the age where they run off and play together in their rooms for an hour while the oldest tells me to leave them alone because they're playing. I am so thankful to have them so close in age. It all worked out. I love both kids to death and have a different relationship with both. My daughter has the mommy and baby bond which is so wonderful and my older son is really my little friend. who I love to hang out and play with.
Hang in there 2fast. I promise that this will all be worth it. Don't be too hard on yourself in the next 9 months, you will be doing the best you can. Your bond will not be broken and he will not hold anything against you. Heck, he won't even remember most, if any, of this. It will be like his younger sibling was just always there! What a cool gift for him.
And plus you always have the wonderful people on these boards for support. They got me through my 2nd go around!!! :)