You touched on a number of areas....
-Self-imposed isolation
-Self-esteem issues (You do not value yourself enough)
-Uncomfortable feelings in social situations
These things aren't fixed in a day, but they are areas you can improve. For your self-imposed isolation.....you need to plan some activities that get you out....whether they be physical activities (gym, biking in park, etc), leisure activities (trip to coffee house, free live music shows, etc), or just trips to explore your town/area. If you have a car you can take some day trips....look up things that are maybe an hour or so away. Ask a friend/family member to go with you...or go by yourself. Set a goal of doing at least 1 'exposure' a day. Maybe weekend days get 3.
Start with that...and work towards the other two things.
-pedagogue