Yes I'm afraid that there is a pretty steep learning curve associated with all 3D CAD systems, I don't think Inventor is much better or worse than other packaged in that respect.
Once you get your head round the way things are done, it gets a lot easier, and dare I say quite enjoyable at times (as long as the deadlines aren't too close!).
Keep at it, you'll soon get the hang of it. There are some text books, but they cost a fortune. Have a look on Youtube, there are a few tutorials, some of which are quite good.