[Lymond]

I would complete IWD first. It's level appropriate, and you should be having fun with it. Some people jump to HOW early, so they can come back and cream the IWD boss that frustrated them in the last game they played. The important thing when doing that is to do it before you get to the point of no return in IWD. I don't remember exactly when your exit to HOW goes away, but if you want to go that route, I'm sure someone knows. I've always finished IWD first, though and been happy to do that.

TOTLM is a judgement call, based on your level. It's not easy, so you might want to wait until you've gone a way into HOW. You must do TOTLM before you go to the sea of moving ice, however.

Another option overall is to play with the difficulty level. It could be that you have a weak party. I did my first game, and I was very frustrated to make it to the final fight with a party that was too weak to fight the final boss. But it happens. So you might want to lower the difficulty level. Later, if you find the game too easy, you can always put it back, or even higher.

It shouldn't be frustrating for you.