I would start by determining what it is that you can't get your camera to do that a different one will. Then you will have some useful guidance.
There's plenty that a D3200 cannot do....not a very sophisticated camera in terms of autofocusing, not that great in low light, crop-frame sensor, slow frame rate, crappy buffer, no high speed flash synch, no remote commander capability, no autofocusing with 40% of the Nikon AF (the Af and AF-D series of lenses which began in the mid-1980's) lenses in the world, etc.. etc..No light metering with manual lenses, no flahs metering with manual lenses... The D3200 is a now $119 used camera here in this town...there is plenty that it can not do...and which many higher-spec'd cameras can do.
Oh...and the D3200 has a crappy, squinty, pentamirror viewfinder. It's a beginner's camera. The D750 and the 5D series were/are aimed at people who want a fully-capable camera, one that can use all of the technologies developed over the last 30 years.
It's time to stop spreading the idea that a $399 beginner's camera is in no way limiting; the people who benefit the MOST from a camera upgrade are typically beginner- to intermediate-level shooters. I used to sell camera gear to the public, and found that higgher-end "serious enthusaiast" bodies and the lenses that are designed to go with them give the beginning and intermediate shooter an IMMEDIATE, and very REAL boost in capabilitis, and in success rates. It is the serious,serious shooters who can often negiotiate their way around the limitations of cheap gear, and yet...funny how the vast,vast majority of serious shooters own high-end cameras and high-0end lenses, and yet, quite often these same people suggest and opine that
crap gear is good enough for the others of the world...
Huh...ever wodnered why so,so many great images were shot with the very-best cameras of their era? Rolleiflex, Speed Graphic, Leica III-series cameras, Leica M3, Nikon F, Nikon F2, Canon F1, Nikon F100, Canon EOS 1...and so on and so on...odd how the vaaaaast majority of 'serious' shooters buy the very best stuff, isn't it?