It is possible that you are experiencing depression even though you can't point to anything as the cause. Some people are genetically more prone to this disease. There can be many symptoms and they can be very hard to pin down. Self doubt and sadness are definitely symptoms. I know it sounds strange but being a goody two shoes can make depression harder to bear. There are so many self imposed expectations and you may feel burdened by your perfectionism. It would probably be a good idea to seek a professional for a physical check up and psych consult. There are some medical conditions that cause these feelings as well as emotional ones. Most physical reasons are controllable with medical care.
Beyond that it can help to get involved with your community and do some volunteer work. Giving to others can make you happy. It just works. You can meet good people and develop friendships based on positive activities. They may have a firmer foundation than friendships based on "fun".
I really think you can get on top of this with some help. You have a wonderful base to build on. Please see someone soon.
I hope you find what you need soon.