I'm sorry you feel so badly about school. For me, my main problems were focused at home, and for that reason... school was my place to get away. The fact that you also have a lot of stress put on you academically (other than just the social aspect) is never a big help. Especially with parents, and teachers, and those around you, sometimes you feel pressured and feel that you always fall a bit too short.
Somethign I've come to realize with time is this. At the end of the day, just try your best. Some people may still think you didn't put forth an effort, but YOU determine whether or not you tried. Even if you try and fail, I think it's more important that you actually took the time to try. At the end of the day, try and be happy with yourself. You aren't horrible, and while it's sometimes hard to think good about yourself... you also need to realize that you aren't perfect, and NO one expects you to be.
In terms of being shy? Well, first of all, that's a quality that some people enjoy. Not everyone is shy, so someone more out going may approach you first... but then again, if you hide from the world, sometimes it just passes you by...
Take it easy, and take your time. There's no way you can wake up one day and decide that you'll no longer be shy. If we could all change the things we see as faults that easily, then life would be a MUCH better place. Work on it bit by bit. If you feel you shyness is what's causing you to act that way, then try and take one small step in the opposite direction. A series of small steps eventually leads to a larger change... There's no rush, in my opinion. And while you may wish this quality to disappear, it's also something that you need to acknowledge. Instead of letting it control you, try and overcome it. Again, small steps... even if it's something simple as joining a club or organization. That will put you in a situation where you're around more people, and the more comfortable you get, you will undoubtedly feel some of that shyness dissipating.
I wish I could take it away, and I wish I could say something like, "Well don't go to school". But I see it this way... sometimes, we have to pull ourselves together and do the things we don't like. Because honestly? I think that you would feel even worse if you decided not to go back.
And if you talk to your therapist again, then explain how you feel about it. Make a list and discuss the anxieties that you have about school. Not so you can convince her that you shouldn't go, but to at least help her understand why you feel this way, and possibly give you some more suggestions as to how you can overcome it.
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All in all, I wish I could just take away the fear and uneasiness from you... but I can't. This is your life, and you need to determine what you need to do to make yourself happy. Small steps at a time... things won't suddenly get brighter... And it's always a bit more nervewracking when you have to go back after being away from it.
Give it your all, that's the best advice I can give you. Push through... I made it through school looking forward to the day I graduated, just because that marked the time that I could finally move away. The anticipation of finally being done was something that I used to motivate me to keep going. You've come so far, you may as well finish it now, right?
Do the best you can do... emotions and depression can sometimes pull us down, and that can take a toll on things in our lives (like our marks or test scores). But at the end of the day, giving in, and giving up can only lead to more regret in the future.
Best of luck, alright? I really wish I could have helped you out more. I'm just glad you have a therapist that you can speak too. She may push you to go to school, but that doesn't mean you can't talk to her about the stresses that it gives you :).