Hi KumquatMaye!
You have my sympathy, 5 nights per fortnight is a lot. It must feel as though you're almost a part-time/part-share mum. Whilst i'm sure that ex misses the children too, I do personally feel that children should have a resident home that they spend the majority of their time. When the time is shared between two people half and half - like yours almost is, I feel like the children don't have a set home. But that's just my personal opinion, others would say you were lucky to have so much time to yourself!
You could do several things: Firstly, why does Exh have them 5 days in a row? could you persuade him to stretch that time over the month instead, say every sat-sun or fri-sat or something? The time where you aren't seeing them will be reduced that way.
Secondly: You could try seeing it as a chance to rest up a little or pursue other hobbies or things. You would have to change your mental view of the situation to achieve this.
Thirdly: You could use that time to plan something really lovely for when your children return home. Bake, make them something really special, plan an outing, etc. If you get involved in doing something great for them, it might not hurt so bad. The antipation of their happiness will make you feel like the good mum you already are, and it will instantly dispel any negative attitudes towards you that exh has tried to implant in their minds. DON'T mention anything you have planned to exh, he will only try to up the ante when they're with him.
I understand how you feel. My ex has the childen three weekends in a month. I don't actually mind too much. But he also takes them for weeks at a time in the holidays and initially I feel like there's a black hole inside me. However, I allow myself to be selfish and I do the things I like that I can't do when they're around and I pamper myself
Remember, the best thing for your children is a mother with a healthy mind. Stress and sadness will sap your energy. You're a great mother, so don't worry when they're gone. They'll still love you! Nothing and no-one can replace a mother.