Hello OP, be kind and gentle to yourself. Of course you are feeling sad. You married this guy hoping that you will grow old together, you loved him (and maybe still do), you have kids together. It is absolutely astonishing how entangled peoples loves become in marriage. When I was leaving my husband I could picture all those roots braking apart as I was yanking them from under me. 😔
I was also unhappy in my marriage and finally decided to break it up after more then twenty fives years. Unfortunately, it was a very bad timing. First Covid started and then series of very unfortunate, tragic things happened to us (to him) one after another. I felt however, I could not postpone it any longer and told him that i wanted to separate right in the middle of that, which was probably the hardest time in his (and our family's) life. This was most definitely the hardest thing I had done in my life. I felt guilty and sad. I kind of ached all over for a very long time. I felt it all so strongly in my whole body that I was certain I would get really sick and perhaps die. It lasted a long time and felt like it will never end, but it did. I would say that the first few weeks after he left were the worst, but gradually the pain and suffering lessened.
I started feeling better, and you will feel better too. I do suggest you try and do anything you can think of that may help. Eat better, go for walks, see friends; I started meditating, going to therapy, I even started reading a bible for the first time in my life. You need to allow yourself to feel all these emotions, to grieve, do not push them away, they are yours, but do not wallow in them. Keep putting one foot in front of the other, keep being true to yourself, keep doing things you enjoy, take care of yourself and your kids, keep looking forward, and in time, you will feel better. I promise. Hugs!