He reassures me every time that it's fine and I'll get use to them and having with us but I just can't seem to see that
The more time I spend on this part of the site and the more I see a commonality in complaints: couple moving way too fast into commitment despite issues with step-parenting. Getting married 2 years after getting together when they are complex issues to deal with relating to the children, it is not surprising that anyone would suddenly realise that it is not as they'd hope.
It takes a lot of time to adjust to step-parenting, as much as for the children to adjust to new important and permanent people in their lives. I am not sure whether the issue is step-parent being in denial and assuming that because they are in love with their partner, everything else will fall into perfect harmony immediately, or whether it is a case of expecting too much too quickly.
The other commonality when things are not good seem to be step-parents getting way too involved in judging the situation, especially the other parent. One thing that I have learnt in the last 10 years is that those people we judge are rarely what we think. I build a picture of my kids' step-mum based on what I'd heard, was experiencing with the children, how my ex had dramatically changed in his attitude since being with her and that picture was not very positive. However, I held back from letting my opinion affect my children or my relationship with my ex. I am so glad that I took that position because I now realised that a lot of assumptions I had made were wrong, or even when likely right, things had changed as the dynamics evolved.
Rainbow, you need to take a step back and try to make it work, which mean probably relinquish some control. Stop judging the mother, you don't know who she really is, what she goes through her head, what she wants, what she really experiences. Saying that a 6yo and 4yo want to live with you is ridiculous. At that age, they don't really know what they want and how could they get to that point of expressing such a wish beyond saying they don't like their mum and like dad and you better after mum applied some discipline (which kids do all the time).
Give it time, but during that time, try to remain positive. It REALLY can get a lot better, but it will take time for you to get to appreciate the children, them to appreciate you, and all of you to learn to share your life with each other. If you allow yourself to believe that one day you will get to really miss them when they are not there, then you will get there.