How long to get over emotional affair

[Speedos]

After the realisation I was having an emotional affair with my 'friend' by discussing my unhappy marriage with him and I guess falling in love, I have gone no contact.

How can I get him out of my head and be able to function normally again?

Have a mantra every time you realise your thoughts are drifting there.
Such as: no good can ever come of this, I need to sort my marriage out, I have no excess emotional energy to squander on fantasies, I cannot conact XXX because, etc.

Rather like going cold turkey when giving up smoking...