Lack of intamacy from fiance?

Me and my fiance have been together almost 4 yrs now and love eachother to bits. But recently we've stopped having so much sex.. We both live seperately with our parents as we cant currently afford a house, so its often hard to find time alone and havin to almost 'plan' when we can have sex rather than being spontaneous is really drivin us nuts and slowin down our libidos. Im 18 and he's 22 so we should be pretty active, but recently we're lucky if we have sex once a week.. and when we do its just cos we think we might as well do it if we're alone so we dont waste the time. My fiance is always tired or not in the mood, and its rubbing off on me too. I sometimes feel like we're more like really close friends than a couple as we're not all over eachother anymore. But we love eachother so much, any advice?? :S