Why stress over someone that won't even check to see if you are alright? Sometimes you just need to distance yourself from people. Nobody is ever “too busy”. If they care, they’ll notice. If they don’t, you know where you stand with them. If they really care, they will make the time.
Very often I read of people with chronic illnesses that continuously stress over the loss of friendships and that people no longer check in to see how they are doing. It's simply time to let those people go. As much as it hurts that they seem to have removed you from their life, and it feels lonely now, you can't hold onto a sinking ship and expect it to float again. Let the ship go. Let it hit bottom and forget about it. It's time to get yourself a new and better ship. Find others that will take the time to think about and care about what is happening with you.
I just recently deleted all of those people that only know me when they want something, from my contacts. That way I am never tempted to just “see how they are doing”. If you are going to let go, you have to let go completely. If I hear from them again, then that's all well and good, but I won't always be the one trying to keep the ship afloat.