I’ve been there. It’s very painful and I know it’s hard but at the moment you just have to accept the feelings because they are a normal grieving process. It sounds heavy, but it’s true, you are grieving. You have to be very kind to yourself right now and accept as much support from friends and family as you can. One of the best pieces of advice I had was to initially take things hour by hour. Hell, minute by minute if I had to! But I can tell you that feeling as if the bottom of your world has fallen out, that free falling feeling, it WILL pass. You will not feel this intensely forever.
It’s extremely cheesy but start to view the intense feelings as weather, let them pass and accept them as transient things. Day by day week by week you will find rays of sunshine peeking through and you will get to the end of the day without even thinking of it.
Some tips for when you feel at your worst are to eat light easy foods, you have to look after yourself: soups, a banana. A boring old smoothie if you have to. Try to schedule time to think about it - ‘worry time’ and write all of your thoughts down, that was a big relief to me. Try to do small nice things and act like a friend to yourself, nice baths, nice smelling stuff, candles, new bedding that you’d like that’s pretty even just cheap changes, a nice plant.
I know all of this sounds trivial and silly, as if this will stop the pain of a partner leaving but these little things got me through the darkest bits. I hope you’re ok op, you will be, keep posting here as the posters here helped me so much when I felt like you. 
