As someone who writes with his left hand I hate them with a vengeance. Haven't looked at them since primary school.
And not unimportantly, a fountain pen requires writing _without_ any pressure on the pen; the ink gets drawn out simply by there being any kind of contact at all, so if you write with a fountain pen the way you're used to writing with a ballpoint pen, or pencil, both of which require pressing into the paper, you're literally doing things wrong (and probably through no fault of your own other than not having realised there might be different ways you need to write).
With regard to the "who taught you to write" hand positions, I wonder if the problem is worse for lefties. My kindergarten teacher actually spent time trying to get me to write with my right hand until my father wrote her a letter clarifying that I was, indeed, left-handed.
I acquired the "overhand" left-handed style at a very early age, which needs a piece of blotting paper under the hand to not smudge everything. This at least is somewhat natural to write with since it's mirrored in the plane of the nib and it's possible to read the word you're currently writing.
From grade 1 to 7 I was exclusively using fountain pens (germany, this is a requirement, so there wasn't a choice) and there was no end of teachers trying to "fix" the lefthanded children in the class.
Writing lefthanded with a ballpoint pen is simply easier than using a fountain pen where I have to either put my hand into a position that cramps after a minute of continuous writing or hover over the paper which also cramps or using a blotting paper, which reduces my ability to keep the last few sentences in mind while writing.
Left-handed writing will also push the pen, inevitably, you will put pressure on it, the light contact required for fountain pens is extremely tired.
After grade 7 I've switched to ballpoint pens, much easier to handle and write. About 4 years ago I managed to find a gas-pressure ballpoint pen which has been the most relaxing way to write.
Unlike normal ballpoint pens, they have gas cartridge behind the ink which expands. They push the ink out of the pen meaning you don't have to press at all to get writing while still getting the benefit of minimal ink usage and quick drying that left-handedness almost requires.
If you're hovering or strongly bending your wrist you've been taught wrong.
After a reasonable amount of experimentation I've ended up just going with Uni Jetstream 101s because they feel smoother than most ball-points but dry fast enough that I don't get ink on my hands. I played around with modding various pen pieces to assemble a franken-pen but having to trim/tape ink refills wasn't much fun.
I'm always on the lookout for other recommendations from lefties. I've seen a couple in this thread already.
I'm genuinely curious why many lefties have trouble with fountain pens, not trolling.
I also use a children's fountain pen, the terrific Faber-Castell Scribolino School Fountain Pen, which I find more comfortable and more fluid than fountain pens which have cost me almost 100 Euros. I highly recommend it, you can find them for about $20-$25 on Ebay.
If you hold the pen in a very curled grip where your palm is right next to the nib, then I can see you may have a problem, but I've found that nowadays, most modern inks dry quick enough and don't smear as easily as the inks I used in school 30-40 years ago.
Also, I learned to write slower with a fountain pen, which is probably better for my mental processing. It also means I make less mistakes by focusing on my writing word by word rather than thinking 2 or 3 words ahead while writing.