Disclaimer; I'm new to the community and the philosophy (for lack of a better word) of poly, so I can only speak from a standpoint of personal opinion.
That said, two things come to mind, both of which have helped my wife and I in our exploration. Firstly, do some serious emotional thought-experimentation and introspection. Play out the what-ifs in your head and probe your emotions. How would you feel if...
Be thorough here, and you'll have yourself a decent emotional foundation for moving forward.
Secondly, communication. Everyone else on this forum will tell you that the #1 best thing to default to in relationships, be they mono, poly or some other geometry, is open communication and honesty. Sit down (or email, text, etc...) with S and be open. Lay down your concerns and explore your insecurities, then work to overcome them. That's what this is all about, right? Forming new loving relationships and growing as people, no?
Anyway, that's my two cents