Well, you play so many fewer hands live that there is a lot more variance, decisions are harder without a HUD, going down a few buy ins has a much bigger impact psychological impact (for me anyway since it's a lot of money for me), and plenty more differences. But to identify your problem we need to know what kind of mistakes you're making that are costing you.
The best way to start is to begin labeling your opponents based on how they play, and play against them appropriately. Identify the ones who call too much, the ones who fold too much, or bluff too much, or never bluff, or anything else useful that you can find out. That much info is enough to start winning money from them, but after that you can start figuring out what cards they play and which boards hit them best and which miss them and make plays based on that as well.
Oh and as for differences in tourneys and cash, stacks are usually shallower in tourneys which simplifies a lot of decisions, and the fact that you can't just reload when you get stacked changes some decisions. I can't really comment much more than that, because I never play them and haven't learned much about them at all.