Coming in late here, but...
I had my twins 17 months after I had my firstborn singleton. (Now, almost 11 years after the birth of my twins, I'm having surprise #4 in march).
Twins are a lesson in compassion and...humility. You will be able to keep your AP principles intact, and it won't be that hard. What is likely to be very hard for you is to learn how to navigate a different way of doing it than you did for your singleton. You will need to learn how to seek and find help. You will learn that sometimes you're not going to be able to meet every need right when it happens (though if you want a secret? This is not a bad thing, and you will learn it earlier than many people do, and your children will be well served for it). Yes, you will sustain some loss of IQ and brain damage from sleep deprivation. Depending on what kind of an AP mom you are, you may find yourself growing in compassion for other mothers who didn't quite do things the way that you did. :)
Keep in mind that *every* one of your children is an individual. Neither one of your twins will have a personality identical to your firstborn. Don't get me wrong, the volume is going to be hard, but there's no guarantee that either one of the new babies is going to give you the same challenges as their sibling. OTOH, yes, this is going to be a time of great sacrifice for you personally. But things *do* get better. I was in a slightly different situation because after the birth of my twins I had 3 children under the age of 2 (and thus, still not firing on all cylinders from the birth of my first, so I never got a breather which I then had to readjust to--but boy am I staring that in the face *now*). But even for me, once the boys (singleton is a girl) hit age 2, I felt I had some breathing space again.
I would encourage you to get involved with your local Mothers of Twins Club, even if you don't like non-AP groups. I kind of steered clear of that for awhile because of the AP thing and I think I did myself a disservice (and guess what, while there were 'norms' in that group, there were plenty of AP folks and blends too). Sometimes it's good to have local support from people who are going to "get" what it's like balancing life with twins.
I've never had to deal with a difficult breastfeeding relationship (yet, keeping my fingers crossed, I guess we'll see) but I was able to nurse all 3 of my kids at once (well, not LITERALLY at once obviously) with no problem. It can be done. I have heard anecdotally that sometimes it's easier to BF round two even after difficulties with round one since you know a little more of what you're doing and so does your body? But it might be a good idea to find an awesome LC who is very experienced with twins.
You're in twinshock right now. This is normal, this is okay. This has nothing to do with how awesome a mom you're going to be to these little ones. You also have a lot going on in your life. You need support. What groups can you call upon to help you? I hate asking for support, I don't like being vulnerable, but I had to with my twins, and it really made me better all around I think. It's the hardest thing, asking. But realistically you're going to have to do it. I think a MOT club would be one place to start. Do you have a church/religious community? Group of friends? Parents of your son's friends? It's hard, but most people really love it when you allow them to help you and have joy in that way. YOu'll tighten your community network. It will feel awkward and weird if you're not used to it at first, but it's an invaluable skill to learn for the long run and more importantly to model for your kids.