How you're feeling is absolutely natural, I felt that way and I was the one who ended the relationship with my ex. I felt like he'd died but worse and I went completely to pieces. I went to parties with friends and disappeared to a quiet room and sobbed at the couples there, or music which was playing which reminded me of him. I think it's something that has to happen in order for you to start again. I even asked my ex back but he refused and I'm so glad he did because I've had 18 years with someone so much better for me.
Allow yourself to fall apart, ask for help when and where you can - if you want it - and take each day at a time. Eventually you'll find yourself again and be stronger for it but don't be too hard on yourself, as others have said you're grieving and you need to work through it