Fredd mate, it's probably the worst feeling there is. People who've not been through it simply can't understand. Having someone that you loved with all your heart and with whom you've carefully constructed your life and planned your future, turn around and say they've moved on - it's truly, truly horrible. I won't go into details here (I've probably told the story here before anyway) but my first wife left me and it ripped me heart out. So I do know something of what you're going through, I really do.
I've said it before, but the best advice I can give you right now is 'don't feel bad about feeling bad'. That means, you've got to accept that you're going to feel properly shit for a considerable time. Some people take weeks, others take months, for me - I took around 3 to 4 years to REALLY get over it. The horrible, painful hurting was constant for around a year and then it came in waves.
If that had carried on much longer, I don't think I'd be here now. I found myself seriously thinking about how I could end everything. At least once I went to do it but thankfully I couldn't.
Thankfully, your sub-conscious brain takes over and says 'enough' and then you start to feel better. But even then, it can be a rollercoaster. Some days will be fantastic. You'll feel positive, you'll feel energetic, you'll feel ALIVE! Other days, you'll struggle to crawl out of bed and not cry like a baby. Some days, you'll feel both extremes and not know what's going on!! Sadly, there are NO short cuts. You have to go through this and man does it hurt. And I'm talking about PHYSICAL pain too, not just emotional. My chest felt tight, I lost all desire to eat along with about a stone in weight. It's a marathon, seriously.
But here I am. I've been re-married a year now with someone who truly loves me and who I love even more than the person who nearly killed me. We're planning a family, we're as happy as two people can possibly be and I promise you I'm a far stronger person because of it. Not that I'm saying this makes the pain 'worth it' somehow - of course not. It's horrible. But just to tell you, you can count on this too, the World will keep going on, the sun does get up tomorrow morning and just as surely, YOU will bounce back from this a better and a wiser person. I promise.
But - that's all for another day. Right now, you've got a long and bumpy road ahead. Good luck and if you want to bend someone's ear, I'm sure any of us would be glad to help.
PS - one last word of warning. And I hate to be the one to tell you this, but it's something that I wish someone had told me. Here it comes...... don't be surprised if Neels moves on and 'gets over it' quicker than you do. Seriously, I don't know what it is about the fairer sex, but emotionally they just seem to cope with all this stuff better than us blokes. Wierd isn't it? They amount of times, I've seen this is scary. Long attached couple break up - both are emotional wrecks at first - then bang, out of the blue the woman has a new flame on the go while her old partner is still a total mess. It's hard. But it happens. You won't want to think about it, but just be ready IF it does.

Slow down...