Insecurity and jealousy are big concerns for a poly relationship. One way to help your partner understand is to make an analogy to having children. If you have another child, it doesn't mean that there is something wrong with the first child. It doesn't mean you love the first child any less. It doesn't mean you will leave the first child when the going gets rough.
I would suggest taking it slow and let your wife adjust slowly. Talk a lot about your wants desires and feelings. It is ok for her to feel jealous. She can work on that feeling by addressing the insecurity at the root.
I think having a support group helps. There are a lot of monogamous people with polyamorous partners. Talking to them can help deal with feelings and coping mechanisms.