We've had...
this conversation several times. There's a good thread hidden away on it here. I am a technophobe so will let the techies provide a link for you.
As historical novelists we all have this problem.
It doesn't matter when you set your historical, you are writing for modern readers.
You can't possibly know, without Dr Who's Tardis, how your characters would have spoken, but you need to give a flavour of the period.
If you've really researched thoroughly you will have some idea of the mind set and patterns of thought of the time. Those of us with written material to access can make lists of words and expressions commonly used.
All you can do is write your dialogue to give a taste of the period. You do this by using a few of those words and expressions within what you think is their patterns of thought and speech. This does not mean using modern formal speech constructions.
Fashions in historical writing change, but at the moment it is not acceptable to write Gadzookery. That is scattering your dialogue with quaint expressions like Zounds! Gadsook! Prithee etc.
What readers, and more importantly, Editors and Agents seek, is good plain English, coloured with images and the occasional expression from the period. This means that you don't describe a wind like a jet plane when your novel is set in the 17thC but you might use images with religious or natural connections. In your dialogue you use contractions where your character would. So family and friends get contractions, talking to the chief priest might be more formal with few contractions.
It's a tricky thing to get right. I had the judge in one competition tell me the language was too formal and stiff in my short listed novel. In an unpublished novel competition two short listing judges praised the 'authentic sounding dialogue' in the same novel. Listen to your crit group if they are all complaining and loosen up the dialogue.
Why don't you post a bit in SYW, the historical section for us to see if we can help?