IMO better than a demo/paper account is to trade real but very very small money, eg R = 1/20th of 1PCT of your account.
For me at least, the psych/neuro-motor difference between risking 0 money vs risking real money, no matter how small the $, is huge. Demo/paper just does not feel real.
My risk-sensitivity function is something like a utility function in the zone btw near-0 and ~1/2 % risk, then goes convex as money risked increases. Perhaps this is a typical experience ?
Though just to get used to the mechanics of trading, demo/paper may be best, but only until the mechanics are a reflex.