I'm not an English-speaking native - my native language is French. It's what I grew up in, and I know it affects my use of English. Especially if I work when I'm tired, whereas my translations are sometimes done word-by-word and thus completely unidiomatic *facepalm*. But I have a beta for it, and she knows what to look for XD. I use awkward syntax at times too, and I know I sub prepositions for others at time. Overall though, I don't think my English is horrible. I've had many, many comments how I have a nearly perfect American accent... while other people know de facto I'm a Quebecer (back when I was working for Bell Canada, some English-speaking clients *fucking hated it*).
I try hard to keep to my stories' context. Example: I'm currently working in a Viking age setting. I've had to adjust to the dynamics, the areas, what pushed their berserker buttons (literally!) and thus, the slang. I'm dealing with gay main characters, but I can't exactly use "gay", "homosexual" and other modern slang, at least in dialogue. Even during narration I'll opt for other words, because I don't wanna break the context for my readers. The concept of homosexuality was known, but not as we know it today; for instance, they made a clear difference between passive/bottom and aggressive/top. It's what I'll be using a lot more soon, especially since their personalities is tied up to those concepts.
I think it can be both easier and harder if your native language isn't English. I work in localization - this is the exact job I have: to make sure the translations are suited for the target, located in a different area, speaking in a different language, can relate to the context. Yeah, you can talk about George W. Bush and Clinton here in Quebec, and people will relate. But it'll be twenty times more effective if you refer to Jean Chrétien (ex Canadian Prime Minister) or Jean Charest (current Quebec Prime Minister). This is why I love the Quebec version of The Simpsons, because it uses both foreign and local references... at the right time. It's balanced.
As for spelling... well personally I go with American English even if I'm Canadian. I don't have a defined target audience, and the differences between American and Canadian English is mostly that we use some UK spellings. So grey/gray... I'll try to get set on ONE variant, otherwise I'm comfortable writing both. I think they actually vary within my stories. It's something I can pick up on my own, because inconsistencies get to me. So if they're consistently spelled one way in the series, I won't even notice it. But I'll notice if "colour" is used once when "color" is used throughout the chapter or story.