I'm not okay when we're not talking

We haven't met. We have been dating on and off for a year. We have had our problems which I will explain another time but currently our biggest problem is me. I panic when he doesn't respond in 2 mins at least. When he leaves to shower or to go to sleep I panic and I get sick. My stomach churns and I get hot flashes. I'm clingy and terrified. I wake up early in the morning and wait hours for him to wake up. I spam him with messages and calls because of fear he's ignoring me or talking to other people. Or he had another seizure. Every time I close my mind I see images of my messages being seen and ignored or him sending me messages telling me how awful I am. I don't know what to do and I know he's getting fed up with all of my issues. I don't know how to save us. I'm scared the anxiety will actually kill me.