Everyone has to find a meaning for life. It is easy to live without purpose. What should you be doing, is not just a question for you but for everyone. Existentialists believe that depression comes from living inauthentically, which simply means that you are not living in a manner that is correct for your uniqueness. Children, when given the freedom to do so, automatically do those things that make them happy. When we force them to conform to our expectations, we take them away from their natural bliss.
What made you happy before, perhaps long before, will make you happy again. I hope that it doesn’t sound too simple to say that you should get back to your roots. Your situation is complicated by your illness. You may be limited to some degree. You should discuss with your wife your feelings of loneliness but you should not be dependent upon her for your happiness. You should suggest to her that you two do more as a couple.
I don’t know your feelings about religion or God but many people find that their relationship with God prohibits them from feeling alone. You can read about divinity in a general way without committing to the dogma of a particular religion.
You may consider doing volunteer work or engaging yourself in helping others in some less structured way. Many people find great purpose and meaning through their acts of kindness toward others. You have asked me for suggestions and I can only be general in my response because I know very little about you. If you were my client, I could be much more specific and that leads me to suggesting professional counseling for you. A counselor would be able to probe your life and make many very relevant suggestions to you about changes that could bring happiness into your life.
I wish you continued medical success and I hope you find the happiness that you truly deserve.