OTT said there was 'something wrong with your S's'. I imagine a more useful way to put this is that there's something Spanish about them. The Spanish s is typically (in Castilian) pronounced with the tip of the tongue; the technical word is 'apical' - but don't go around saying you've got an apical s because only students of linguistics will understand you.
If you want to change this, slide your tongue very slightly forward. But be careful: a lot of Spanish speakers do this, and take it to far, producing a /θ/. Just rock the tongue, so that instead of a | it makes a \; that is, the angle between the tip of the tongue and the back of the gum is less acute.
But, as others have said, accent is something that doesn't matter unless it interferes with communication.
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