I'm thinking of living a second life behind the back of my ex-gf
I am not sure I understand what you are saying. Do you think it's possible for you to do that?
I've been working on this for quite long time
If you truly care about your ex, you probably shouldn't do that.  It's wrong to live a lie behind the backs of those you care about.  If you've finished moving on from that relationship, then I'd suggest you cut off contact rather than just lying to her.
Why can't I? I can have a hobby too.
There is no reason for you to do this and be deceitful. No one will gain anything from this and you will look like a bit of a jerk.