It's not just...well... its jealousy and envy, by those definitions. And resentment. It's not so simply explained.
Thus-far, I haven't spent any time with her. Talked to her twice and had some fleeting messages passed by way of our boyfriend. And as mentioned, I admit that I don't know her all too well. By way of him explaining the situation between them, there are some, difficulties (for lack of better word) going on. Be it money, or what she provides, I don't really see what shes contributing. (And yes, I know it's not my place to judge... but it's my natural instinct).
Logical and defensive emotions slowly tumble into resentment, as it seems that she's not making things any easier between the who of them. Whereas I feel, unfortunately, justified (I'm very much the care-taker sort... I meld to what a person needs). And then that turns into being indignant. And a lot of "why" questions.
Why is she the primary when...
Why am I stuck with the "short end" of the straw when...
A lot of "why does she ____ when I go out of my way? Especially when, she may, in-fact not be hurting the relationship, but shes blatantly not helping?" type questions. It's all a very hard thing for me. And I've explained some of it to him, but I don't have the gall to flat out say the above.
Usually, I get the, "I'm a grown man, if I didn't like it, I'd change it" though he's expressed it's tiring and stressful.