Adding to bhg's response, if the inter-pane seal has indeed gone then this is usually manifested by fogging or worse, at least in some partial area. If the door is single glazed then you should urgently get double glazing. In either case, you should choose glazing that is thermally biased. One of the earliest commercially available types was Kappafloat by Pilkington but there are now numerous similar makes available. A double glazed unit will have an "In" and an "Out" side and when fitted that way will actually bounce heat radiation in as well as insulate with the cavity - on a mildly sunny day the inside will feel decidedly warm, at whatever time of year. We have these throughout in all windows and I have a nice photograph showing when the windows were fogged - on the outside ! The glass was, on the outside, colder than the air temperature there (hence the condensation) whereas on the inside that pane was not greatly different from the 22 degrees or so throughout the house.
Also, to have the glass replaced should not be a significant cost and, in particular, it should be possible to replace it in virtually any door. You will likely be told by tradesmen that they want to replace the door - this will have more to do with convenience/profit for them and/or lack of imagination. You should not have to give in to this. If you are good in DIY then you will be able to do the work yourself(ves).