FSX ATC is no way comparable to real life ATC. I think of it as a glorified flight watch. The reason you get so many deviations is they give you a heading then for some reason the heading ATC gives wont keep you on track (be it wind or something else I don't know.) So you drift off course so the multiple deviations is the ATC getting you back on track. Its just a weird glitch of FSX.
Using the C172 to practice is a great choice as, I just wish more people would do that rather then jump straight into a 747

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IFR is generally only used when weather is foul and you cant see landmarks or if your flying above FL180 (in the US anyway.) There really aren't any rules as to when you can or cant fly IFR but if there isn't a need caused by weather or if you want to do some training in IFR then why put extra stress on ATC? Even if they aren't real

And to save the aircraft "Cold and Dark" set up the aircraft with all systems off and save the flight and check the box to save it as the default flight. To start in a gate choose the starting position when you pick the airport. The drop down menu should be in the lower right corner when you select the airport to start from.