Anyways,these are a few of the prominent problems I've encountered. Honestly, I don't think there's any easy fix to any of them. IMO, perseverance and working harder than a native speaker is the only solution to improving, although having a good editor is an immense help.
1. This is one for the editors, but some volunteer editors will pick up certain things, while others won't, so I'd try a variety of editors, to see which are good at prepositions, as well as having at least a one American and one Brit-English one. Even if they don't catch much, having that expertise on hand can be a godsend.
2. Rather than Google, I'd do a variation on the pillow dictionary, and use a wife, girlfriend or simply close female friend, butter her up on occasion, and use her as a sounding board. As long as she feels appreciated, rather than merely being used, most won't object to the odd-hour phone calls.
3. Editors are not reliable sounding boards. I'd suggest you find (preferrably from your readers, if not then among friends and associates) some beta-readers, who'll ignore typos and obvious corrections but focus exclusively on 'perceptions', things which strike them as oddly phrases, or which raise red-flags. Once you've done it for a while, it'll slowly start to sink in.
4. I've run into a similar spelling problem from my early years reading. I tend to spell longer, complicated words with British spelling, while for everything else I use English. Like you, I had readers comment on it, while my editors mostly ignored it. I started doing global search and replace operations, burning into my memory which spellings were American. That seems to have worked, though I'm sure I lapse without realizing it. If you can identify which British/American words you switch, a quick Google search should quickly address it.