Hi IWMI,
I'm another who just wants to pass on my support. I was in your position 1 yr ago (1st time he left) and 6mths ago (the final nail in the coffin!).
Like inthemistoftime says I don't think there are any rules. Just try and take each day at a time. Try not to worry about things tomorrow/ next week/ next month that may or may not happen, just get through each day.
My exh still has a key to house and lets himself in when it is his time to see our DS. I would just be slightly cautious in changing the locks, if he jointly owns the house then he is still entitled to access, however crappy that seems it unfortunately is the case. Get the keys back if you can, I asked exh for them but he refused to hand them over.
Changing a few things/ moving furniture etc is a great idea. I shifted a few things around, took down photos of him and put up loads of cheery photos of DS.
Pack up his stuff if you can face it - I found the process of doing this very helpful and was quite delighted with the amount of wardrobe and drawer space it gave me (see....there are some pluses!!).
I do pretty much all communication with Ex over email as I still cannot bear to speak to him. Obviously I am polite in front of DS but the absolute bare minimum (it is relatively easy as DS only 1yr).
It does get easier, honestly. I know it may not feel like it at the moment but the pain/ hurt/ humilation/ anger etc. does diminsh.
You said you had been to relate with him....did you find it useful (obviously not in terms of the marriage, but in terms of your own thoughts and feelings?). Exh and I also went jointly and although it didn't help him (her kept on seeing his OW) and it didn't help the marriage, I did find it useful, to such an extent that I started going on my own a month or so ago. I was still struggling with the anger and bitterness towards Exh and spending too much time dwelling on things - the individual sessions have helped me move things on a bit. Maybe think about that as an option (you don't have to have the same counsellor - in fact I specifically requsted someone different).
Also try and keep busy, obviously you will be with 3 DCs (!) but in the evenings and during the weekends when your Ex has the children try and see this time as an opportunity to do something for you....see friends, have a bath, go to the gym, do some gardening (I have a very neat garden for the first time in my life!).
I'll have a think about some of the other things that help me and post again later.
Take care