OP, I can't tell you how much I relate to what you're going through. My situation is similar - my ex-husband has recently moved in with someone I once considered a friend, and who I worked with for years. I don't know when they got together (he won't tell me), so it's unclear if they were seeing each other before we split, or not. He's become for her what I always said I needed, but he was never willing to do that for me. It's absolutely devastating. I can't believe how long it's taking me to process my feelings about all of this - and to have wasted 11 years of my life supporting and taking care of someone who couldn't even be bothered trying to fix things and moved on to someone else... but here's what I've been trying to do -
This processing makes us question our reality, to relive what's happened and how much we've lost. There were good times- of course there were, and those are still yours too. But it can be easy to forget the reasons why it ended in the first place. You weren't happy, and he wasn't willing to fix it - by the sound of things. That's important to remember. Whatever he is now, whatever he is doing, try to center yourself and think about you - who are you, what do you want and need from a partner? Remember that everything wasn't perfect, remember that there were legitimate problems, and reasons that it ended. Try and come to peace with the fact that he wasn't willing to give you what you needed, and that you deserve better. I hope you can find peace - I'm still trying, but I often have to remind myself that this is to goal. But you also need and deserve to grieve this loss- you've lost not just a person who was one of the most important in your life, but also a future that you saw for yourself with him. That's important - you need to recognise that loss and grieve it. Give yourself time and be gentle with yourself as you process it.
You matter more than what you feel right now. Your happiness, and having a partner that respects, loves and supports you matters. He wasn't that for you - not enough anyway. Not by the end. Try to find peace. You're not alone in this - there are so many people going through the same thing. I hope you can find peace very soon x