TRye, I also grew up in the South, so I can understand some of the conflicting emotions you have been feeling. (As evidence if that upbringing, your username reminded ne if the city of Tyre in whose shores Jesus preaches in Matthew, Mark, and Luke.)
Do I recall correctly that you are a mother? Ok, so let's look at that. You married hubby and bore him a child. When child was born, did you love the child? Yes? In order to love the child more, did you take love away from hubby? Did you love him less? No? So you love him as much as before the baby or more. Does this not already indicate that within you is the capacity for great quantities of love? When you had child #2, didn't your love and hubby's love grow?
You are right that it is a hard and scary thing to see this world that falls outside the safe boundaries we grew up with. Those of us conditioned with certain backgrounds will automatically question our worth as humans. This is part if the growth pattern. There is hope that you can stretch beyond the scary and plant roots not on the rock but on fertile soil.
It sounds like your husband really is looking out for you. How great a person you must be to evoke such devotion. Be proud of his love for you and the fact your do deserve it. As a devoted husband myself, do him a favor for me--let him adore you for your awesomeness and be glad. If you don't agree that you're awesome, too bad--it's his call to make,awesome wifey.
That said, your fear and conditioning has made a mess. Hubby has tried to help, and that's a good sign. Hiding your great love under a bushel isn't a fix. Covering your capacity for love hurt this new woman and seems to have upset your helpmate. There is, however, a solution.
Talk to your husband. I was terrified when I first tried vocalizing my feelings about polyamory, so I know all the "what its" running through your head. Your husband sounds nurturing and supportive, and that's a wonderful start. I'd bet he knows you better than you give credit. You may even find out that your poly nature is more news to you than to him.
You mentioned past mistakes for which you seem to carry guilt. Let it go. Hubby has you back and is right there with you. On some level, I wonder if thise scenarios were a nacent poly heart forming in your heart. In any case, now is a time for the both of you to grow, and you may grow together even as you grow to love others.
Once you've talked openly with your hubby about how you feel and what you fear, then comes the really scary part. Ask him what he thinks you should do. Yeah, it's scary. And it's your heart and head that have to decided. However, laying your worries at his feet and asking what to do is a huge sign of how essential he is in your life. He may not be poly himself, but he may be able to help his poly-prone wifey find the answer that heals her awesome heart.
Pax vobiscum. *hug*