if you feel better taking half the amount but twice a day, then that is ok as long as your doctor is aware. It is supposed to last a full 24hrs if taken once a day.
25mg may very well be effective for you, i would try it for at least a month and see how you do. some people need only 25mg, whereas others need 200mg! everyone is different so all i can say is try it!
let me know if you have any other questions,
yes, it is.