Confidence definitely helps. Insecurities tend to keep us from growth. I know it seems biased to say that learning to be at ease with your wife's polyamory is "growth" but I think anything that we endure which is difficult helps us grow when we come out on the other end.
GS's advice is good for people who see sex as "just" sex. But if you're not one of those people, perhaps the opposite might be easier? If you can get comfortable with the idea of your wife being in love with a second man, maybe it will be easier to handle her expressing that love physically? To that end, RP's suggestion of getting to know the guy could help.
Sex is different for everyone, but the bottom line here is that if you want this all to work out, you will need to learn to cope with the feelings and hopefully turn them into something positive. Not saying it will easy, in fact I can pretty much guarantee it will be difficult, but love is worth it!
“As I am sure any cat owner will be able to tell you,
someone else putting you in a box is entirely different
from getting into a box yourself.” —bisexualbaker