Please help me work this out 
Ds goes to exP on alternate weekends. Sometimes exP wants to swap because things have come up. Then he wants two weekends in a row in return. Eg: can't do the one coming up, so wants the two following.
On paper of course this is fair, especially if you think in his terms of having ds two weekends per month. In reality, to me (and ds) this seems unfair; exP can't do his, so can't we just change our sequence and then continue onwards?
It's not an official arrangement and mostly civilised. I am very flexible, if I have made plans I will change them, as this makes life easier for all, especially ds. I'm not being a martyr, just as a single person, it's easier for me. Also, ds (11), loves his dad but would always prefer to be home and it takes a lot of encouragement to make him happy to go each fortnight. Changing adds to this stress, but I don't want to deprive exP from his ds either.
So AIBU?