Divorce: why do I feel so sorry for my ex-husband? And guilty?
I expect that when you married him, you loved him and pictured a life together. When you go your separate ways from someone, it is not always because the love is completely lost, it's often because for some reason or another the relationship no longer works.
For any number of reasons, your feelings for now will be complicated, especially as you sort out what your life was going to be like when you were planning it with him, and what it is actually going to be like now. Having that shift can induce feelings of sadness, guilt, unease, a ton of different emotions.
It will get easier over time, especially as both of you begin to move on and develop routines, friends, lives outside of the life you had with one another. Part of you may always feel sorry or sad that it didn't work out, but like the others said that is what makes you human and having compassion for others is not a bad thing!