Talk through your fears with your wife and ask her what she wants you to do next and why. It will help both of you to understand the source of her enjoyment and what positive feelings she gains from it. Shame mixed with excitement is common at the stage you two are at. Share that with her and explain your fears; it will help alleviate them. You obviously love your wife very much and are willing to do anything to bring her sexual pleasure. Nothing to be ashamed of in that. Keep the communication open. I think you'll feel more comfortable in panties and soon will be ready to suck her lover's cock and swallow his hot load in front of your wife.