You need to do a few things to start to detach. Look at your STBXH as the man he is and not the romantic hero in your head. You have to realise that the man who is moving on with his life is very different to one you fell in love with.
Do at least one good thing for yourself every day - whatever it is you love - a bubble bath, favourite food, lighting candles. Consider redecorating or moving the furniture around - make your home entirely yours.
Think back to all the things you loved to do before you met him but that you stopped doing because you were busy focussing on him. Write them down and choose one of those things to do this weekend. The idea is to rediscover who you are. Dig out the fun loving woman that so often gets buried in a relationship. Be selfish!
Write down all the things you've always wanted to do and plan to do them, one by one. They could be small things like joining a gym or going to a pottery class or big things like learning to ride a motorbike or going to Nepal! Have lovely goals to look forward to.
Write down the things you're scared of doing now you're on your own and figure out how to get over that fear. I was scared of going to the cinema alone - really daft - so I picked a film and off I went and I loved it! I actually prefer to go to the cinema alone now!
Plan everything. Use a diary and fill it. Get in touch with friends and suggest going out for drinks. You may find, like I did, that you need to find new friends. My friends were all coupled up and I wanted some single girlfriends so I joined City Socialiser and Meetup to get myself out and about (I was 45 when I found myself sacked from my old life).
When your heart is breaking at 4am turn on the light and write - let it all out. Those words will spew out and it doesn't matter if it's all about hate and sadness. It doesn't matter if they don't make sense - just let those thoughts and words out. I would read what I'd written the next day when I was calm again (much of it was illegible) and then bin it. I found that after a month or so I was scrawling less and hating less.
It takes determination and time but you need to become a whole person again with your own life and your own interests and while you're putting yourself first your feelings for him will start to fade, they really will, and one day you won't cry when you think of him and then you'll find you don't think of him at all.
You will probably have to force yourself to be shiny and bright - the fake it till you make it days - on other days you'll bounce out of bed and embrace the day and there will be dates when you can't and you just sob under the duvet but you know that tomorrow you'll be up and at 'em again.
I've been there, I've done it and I've got the T-shirt. I didn't want any of it but I did everything I've written above and it worked. It's not easy, it's not quick but it's better than pining for someone who hasn't given you a backward glance and there's no reason why you can't be the one who's careering ahead with your new life. Good luck!