You are not alone in being a blood-donating loser.
My first attempt, the blood would not flow out of my arm, no matter how much I pumped that little rubber thingy with my hand - ended up with a grumpy nurse (like it was my fault!) who ripped the needle out in disgust after I had only filled half a bag because I was "too slow".
Second time, all going well until a nurse dropped a bag of blood from another donor on the floor and splash, blood everywhere. Too much for queezy me and my blood pressure went through the floor. When I came round, I had 3 nurses, my mum and my sister peering down at me in concern, a fan blowing on my face and my feet up in the air. They politely suggested I wait a couple of years until I try again.
Wish I was a bit more robust so I could be a regular donor.