I felt the same about adopting Mika so soon after Thandi died (about 3 weeks). It really was the guilty feeling of "did Thandi mean that little that I'm adopting another dog already, and why did I do that, and will I even like her".

And yes, it was all of that, and I ADORE Mika and the second dog I adopted on Saturday. But I love them differently to Thandi, and that will always be so. No more guilty feelings.

Give yourself time, and both of you will grow on each other before you know it.