I know the double-edged sword when the kids seem ok - we SO want them to be ok, but there is a part of us that wishes they were "on our side," proof to the world and the ex (ex, now right? not STBXW...sorry, it was hard for me to transition, myself) that this is all so very wrong. It is hard to share their happiness when we aren't a direct part of it.
But, you are. They can have happy moments with each of their parents BECAUSE you have worked so hard to do all that you can for them, BECAUSE you sacrificed so much to make sure that you are the best father possible.
Plan a trip to the Islands (can't think of which speaks French off-hand), take the kids swimming with dolphins, etc. In short, keep your focus on you and your time with them - make plans. And let a little glimmer of gratitude in that they are safe, cared for and loved when they are with their mother (even though we all wish that our ex's were better parents - most seem to at least be trying their hardest to be as good as they can with those relationships, given the flaws they have).
