I am sorry for the loss of your cat, but happy for the new addition to your family.

It must be hard to see a bright spunky cat and know this is not your friend of 19 years. The relationships of new pets is not the same usually. They cannot replace an old friend.
Grief is something that can be faced and dealt with. It is not a condemnation but a chance to commemorate that relationship and come to terms with the loss of that friend. It takes time but it can heal. What is so difficult about grief, is that we are not taught how to grieve in a healthy way so we can accumulate unmet grief in life so every new loss brings out all the old losses we have yet to reconcile ourselves too. This inviting grief and facing it can also deepen our experience in therapy as well but tuning us in to the things we have avoided during our lives, but need to face sometime.
To me there is no pill that will make me happy, stable yes, but happy no. So when I get fairly stable, I have had to create my own happiness by taking on new challenges in my life like exercise and yoga, mindfulness and volunteering, and just being more involved with life.