I take it that you mean you removed a door which was 44mm thick and that the rebate in your door lining is also 44mm wide?
Your new door is presumably only 32mm thick? So, to avoid having a 12mm gap between your new door and the "stop" in the lining you have 2 options.
1) Replace the door lining or frame, this also means the architrave both sides of the door. Whilst this is not hugely expensive in terms of materilas (Â£30?) it does incur a fair bit of disruption and time. If you have to pay someone to do it the labour cost could be steep.
2) Strip all the paint from the rebate in the lining and glue a strip of wood all round to fill the void in the rebate, i.e. approx 12mm x depth of rebate. Make good with wood filler etc and sand flush with the lining and paint over. With a little luck you wont see the joins.
Can't think of another way but then I'm not a chippy, just a diyer!