J.E.
My kids are 17 & 15 and this is the second summer they will be spending with their dad. They will be gone for 2 months (to another state). When they left last summer we were in the middle of a custody dispute and the unknown was very difficult.
I want them to spend time with their dad, but I didn't sign up for this. Set-up skype and times to talk to them, but stay positive and encourage them having fun. I talk to them about the fun things they're doing.
I don't think it gets easier. After about two weeks last year, I went through and cleaned their rooms, but it took me time to be able to do that. Allow yourself time to be sad, but don't let it take over or take away from your daughter or step-kids. Be aware that they may get jealous of you going and doing activities without them. To them its new and fun, but they expect everything at home to stay exactly the same.
My son made a list of his favorite dinners that he wanted me to make before they leave. My daughter and I made her list of everything for her to bring with. Last year we painted some picture frames and I printed some photos she wanted.
My kids leave in 8 days and I've stayed positive until now. Every night is scheduled with friends and family they want to see before they leave. I find myself taking a mental picture of the little moments. Last year a few days before they left my daughter came up behind me and put her arms around me and laid her head on my shoulder and said I love you mom. I replied I love you more and she said you couldn't possibly love me more. It makes me appreciate and slow down for those things. Change up your normal routine a little. Know that they miss you and you can never be replaced.