Where is the line in the play where Macbeth says that he is already so deep in this that to go back or forward is the same distance?
The lines you refer to occur in Act 3, scene 4, lines 168-170 in my edition of the play:
I am in blood
Stepped in so far that, should I wade no more,
Returning were as tedious as go o'er.
Macbeth has just disrupted the banquet by seeing or thinking he is seeing Banquo's ghost. He is alone with his wife, who has had to cancel the banquet and ask everyone to leave. Now he learns that Macduff has refused to attend the banquet. Macbeth is becoming suspicious of everybody. He tells his wife:
There's not a one of them but in his house
I keep a servant fee'd.
Now that he has murdered Duncan and Banquo he is becoming inured to bloodshed. He tells his wife he is going to consult the Weird Sisters again:
. . . for now I am bent to know
By the worst means the worst.
From this point on he intends to rule by force and terror, and naturally he alienates everyone and makes his own downfall more and more certain.