I am sorry you are experiencing these feelings. I often feel like having your guard up is a defence mechanism...maybe you've been hurt before..maybe you find it hard to trust others because of that experience...or maybe you're scared of rejection...maybe you want to try so hard that it freaks you out and you retreat because of fear that it won't go well. I only say this, because I have definitely experienced it myself, I am sure everyone has to some degree. You are not alone.
As Petal22 said, start small and build up. I often find that asking others about themselves is a great way to fuel a conversation because..well..people love talking about themselves, LOL! At work, whenever someone new starts or I haven't talked to someone recently, I ask where they used to work, their experience, and often you can get a fair chunk of their work and life story from this! Yes, some people are a bit more closed of, but most people will be receptive and happy that you took the time to talk to them. Or - just ask about their weekend or night...you can get a conversation going from this too !
It can be hard but I would also suggest seeing your GP and getting a mental health care plan - it is easier to get an appointment with a psychologist this way.