I think as the others have said, you need to tell the sensei. There is no shame in telling your nage " lets take it slow"
I too was nearly injured on a shionage with alot of umph at the end. My head was tucked when I went down, but the force of the fall bounced my head off the mat. I wasn't expecting him to go that fast and rigid. he was doing the technique wrong too so I could have ended up seriously hurt.
I was more mad than anything. Not in the sence like Grrrrr Im gonna get this guy. But angry because I put my total trust in my nage to practice on me without injury and he treated me like a training dummy.
My advice would be to tell the sensei and all your nages that they need to take it slow and easy on you until you gain the needed confidence to go faster. There is no shame in that and they won't look down on you.
If they have a problem training slow and gentle and they just can't manage it, then THEY are the ones who need to revaluate their Aikido skills.
I can play a guitar. I can play a song lound and obnoxious with all my might, but I can also play the same song so slow and soft it would put you to sleep. My aikido is the same way.
There is nothing wrong with being timid and unsure of someone who is to rough on you. You just have to take it slow and easy until you can work up to the faster stuff. When you are ready, is when you will be ready.
hang in there.