You're getting quite a bit of unnecessary flack here, simply for being honest, imho.
You're undoubtedly hurting. And this week more so perhaps because they're away for the week with her.
Try and keep busy. VERY busy.
When my DC were with their Dad, I grieved in more ways than one and I too felt jealousy, resentment, bitter - on top of missing them and hoping they weren't forgetting about me and preferring his company (while, in my rational mind, I of course wanted them to feel free of the burden and enjoy themselves and be children). It's so hard.
So, I busied myself by re-decorating their rooms - a lick of paint, then popping to the shops for new bedding, new cushions, new lamp - nothing too pricey but basically, making their bedrooms lovely for when they returned.
And as much as it sometimes killed me, i NEVER showed my resentment and my jealousy and I always showed delight at what they told me. Eventually, they told me a lot more and they came to realise that their Dad was a user who took more interest in his step-children.
Be the strong one. Be the one who welcomes them home. Be honest and tell them that you miss them but that you're excited to see them when they are home.
Make sure you have something planned for when they get back - their favourite meal, a film and popcorn night or a meal out. Anything that centres THEM.
Take time to meet your mates - go and do what YOU like to do - try and distract yourself.
In time, they will see and understand who stayed. They will know who did what and you won't have to be the one to tell them. Even if Mum is wonderful with them, your children will eventually know who their rock was.
Look after yourself. It does get better.