3-gatsu no Lion, Doukyonin wa Hiza, Tokidoki, Atama no Ue. Recommendations
Both stories start off with a depressive main character who doesn't communicate much with other people. They live on their own. They are very good at their work but they lack interactions with others. As the story goes on they find that "something" that keeps them going further in life (it's different for either series!)
If you like stories like these, check them out!
My Roommate is a Cat is way more light-hearted and comedy-based, but the protagonists are similar. Both of them deal with social anxiety and loneliness and depression, until others come to their lives and gradually "force" them to open up.