Please Help me, I am freaking out?

So here is my situation. I have been unemployed for the longest. I finally got a job and am suppose to start in about 3 weeks. I have been unemployed for so long that I feel like those 3 weeks are 3 years. I am freaking out and constantly keep talking to myself trying to convince myself that I in fact have a job (for some reason am still doubting myself) and i need something to do to stop thinking about it. I even asked them to send me an employment letter hoping this would calm me down (havent received it yet). what should i do to stay sane because i feel like im losing my mind. I dont know what am suppose to do help !!! anyone?