I have the same problem, I tried so many ways of holding a pen, but nothing works. Like you said, it starts already after writing one sentence or something like that. Made my schooltime really complicated.
How tightly do you hold your pen? That’s a lot of people’s cause of hand strain.
It should be held as lightly as possible. Ideally someone should be able to lean over your shoulder and pluck your pen/pencil straight out of your hand with next to no resistance.