Loosing in won positions

It seams lately that I have lost a number of games in won positions. I come out of a combination up material and fail to consolidate, allowing my opponent counter and even mating chances. I play such good chess then relax and play some "hope chess" moves and loose.

I know time pressure is involved but really after such hard work it seams such a shame.

For a bit I thought it might be that I should take some won positions and practice playing them out. However, I don't think it is a mater of knowing HOW but rather mental laziness. However I think taking some of these losses and replaying them vs a computer might help me.