I'm sorry @Farahdwy
I'm going through the same and it's really hard. I have that same feeling in my stomach too, mainly whenever I have to have contact with my ex, which I have to do for another few months yet.
Some practical things that have helped me:
I belong to a running club. Lovely people and a good distraction and the exercise gives you an endorphin high. Hard to make myself go sometimes but I never regret going. Enter races - they give you something else to focus on. If that's not your thing, find some exercise, ideally with other people, that does appeal. It's as good as taking a mild antidepressant.
Set up a Google sheets doc on your phone and list all the bad things about your ex. Read them whenever you need a reminder of the negative things about him/your relationship.
Book things in - holidays/concerts/seeing friends etc - so you have things to look forward to and to distract to.
Buy a nice notebook and write down the nice things that have happened to you at the end of each day.
Write a journal whenever you need to. I keep a short diary at the end of each day. It helps to get my feelings out and on paper.
Go no contact if you haven't done so already. This really helps to detach yourself from the person.
Someone on here said save a period of time each day for ruminating and crying and being upset. And then, after that time is done, get on with other things. I keep forgetting to do this but it's a good idea.
Hope some of this helps.
If I think of anything else, I'll let you know.