Lulu, i experience the same thing too. When i'm having a good day, that does not mean that im' anxiety free. That just means that my anxiety is a little less and is bearable. But my mind keeps going and going and going. So i too have the same question of how to prevent this. Sometimes i just try to focus on the PRESENT time and try to encourage myself to stop worrying about the FUTURE time. Just enjoy what you are doing, appreciate what you are doing at that moment, talk to yourself about some the positiveness and loveliness of what you are doing....and try to convince your self that worrying about the future is really a waste of time. Also, convice yourself that you have been through this MANY times.....so whatever shall come in the future (if it does come) is bearable and cannot harm you. My homeopathic doctor told me "Just enjoy your present time. If a panic does come in the future time, you will deal with it THEN and only then. Stop worrying about it and wasting your time."
So i'm trying my best to follow this advice. Its hard at times. But it does help! I hope i made sense hehe. Good luck.