Envy in a Poly Relationship

I find myself envious of my girlfriend, and I know this is hard to admit, actually it's always hard to admit that everything isn't going perfectly in a relationship, at least for me it is. But I do envy her, Sweetheart is almost everything I'm not, she's very flirty, fun confident, and above all very sexy. I however, am sweet, kind, adorable  and naive.
Now for Husband he gets to have one of each kind with the two of us, because in the ways we are different we tend to compliment each other's personalities, but for me that sometimes leaves me feeling inadequate  or like I'm not good enough to please Husband. I know it's rather ridiculous especially since Husband loves me, and he loves Sweetheart, And I love both Husband and Sweetheart as well but I still feel like I am not good enough.

I tend to blame it on my social awkwardness as to why I want to be sexy and flirty to Husband so bad, but I always tend to fall short, and both Husband and Sweetheart think it's adorable as to how I try to be flirty.
What I really need to do is just stay in my niche, what I'm good at, but I see their relationship and I think that Sweetheart looks like she's having so much fun being flirty, and I just want to have fun sometimes, I just need to learn how to let loose and know that both Husband and Sweetheart love me for who I am, and neither of them want to change who I am, or how I am. (Although I'm sure Husband would enjoy me a little less socially awkward, but I don't know if I would truly be me if I wasn't so awkward)

And before anyone asks, we all three talk though these feelings, quite often in fact. Because if we were to ignore them, it would just get bottled up inside and create issues that can cause rifts in the relationships. And I know that Husband Sweetheart and I all three want our relationship to work out, because we plan on being together until we are old and grey sitting in our rocking chairs, Husband flirting, Sweetheart sassing him, and me crocheting and thinking both of them are crazy, and that I will love them till I die.