Both of your questions depend on
what you decide, what you choose.
This is because to be guilty of sin
one must choose to do it.
So to intentionally, deliberately
fantasize sexually about a fake person
to get pleasure from it would be
sinful. When we become aware of these
thoughts that come into our heads
we have the choice to either entertain
them or reject them. We should always
attempt the latter.
God can forgive a sin each time it's
done, but there has to be true sorrow
and a resolution to avoid the sin
in the future. If one is obsessed
or addicted to a particular sinful
act, only God knows the degree of
freedom the person had in their choice,
and the degree of guilt (if any)
there may be. But regardless, it's
always good to take it to the sacrament
of Confession in order to be forgiven
for whatever degree of guilt there
might be, and to receive the grace
to be freed of this sin.