what you need is a clear boundary not 500 curves stacked into one what looks like a simple shape. if you have to use those curves and there is nothing else to work with where that comes from then you have to clean that up.
you can use CurveBoolean to clean up most of it but these parts shown below you have to fix manually for sure the curves have to intersect completely for CurveBoolean to work.
after that you can extrude or do whatever you want in grasshopper.
you can also try to use grasshopper for the first step already, but i assume that you would have to manually fix these regions outlined above anyway. this or one of the options in grasshopper can actually mitigate the issue for you.
here some short reading how to do that in grasshopper itself. now go and have fun 
edit:
here i still found a topic which seems to have some good suggestions.
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Thank you for your advice. I’ll try to operate it