I've recently gotten into a poly relationship with a pre-existing couple. I'm actually dating only one of the people in it.
I am guessing you're a man, dating a woman who is also dating another man? You are in a V, and the woman is the "hinge." You and her pre-existing partner are the "legs" of the V.
They've been together for 2 years. I've only been with her about 2 months.
So you feel somehow "less than" because she's been with the other guy longer? You are the new exciting partner. Her excitement with her other guy (their "new relationship energy") is dying down, but this NRE is happening with you and her. Part of NRE is excitement, sexual arousal, but also anxiety. You barely know this woman. You don't know where you stand with her in her life, in her affections. Time will be needed to find out if you're long-term compatible.
But seeing her mention her other partner and make jokes about how they got together so much, makes me feel so worthless, because we don't have a connection like that. We don't even talk similarly. I feel so intrusive and greedy for wanting more. My heart hurts so much. I don't know what to do. I just want attention deep down. But I feel like I'll screw everything up. What do I do, even?
Is this your first time trying to do ethical non-monogamy? Is it hers?
What do you need as far as "attention"? Be more specific. Figure it out and then request it from her.
Do you want more time?
Do you want, not more quantity time, but better quality time?
What would improve the quality? Do you need her to talk less about her OSO? Some new to poly people don't realize they need to think less about themselves as part of a "we/us" and more as just "me/I/myself." Otherwise it can increase a feeling of competitiveness on the part of their partners.
Are you hanging out at her house, hanging out with her OSO? Would you prefer to not be around her other guy? You could have her over to your place, get a hotel, something like that. (There are different kinds of polyamory. There is "kitchen-table poly," where everyone hangs out together, or there is "parallel poly," where you see her other partner never or rarely.)
What else would make you feel more "seen" and secure?