Becca,
I agree with everyone else that this feeling of longing is due to the unfulfilled needs of childhood. When our need for love, attention and attunement is not met when we are children, these needs to do not dissappear, then only go underground. This leaves us with an unconscious life-long quest to get those needs met. I also believe that this longing is a reverberation down the years of our early relationships, when we endured the deprivation of not having our (very reasonable, normal and human) needs met.
I struggled with those feelings for a long time (and still can at times when something gets triggered) and also wondered why I still felt so unfulfilled even after being in a committed relationship for years, decades even. There were two major dynamics going on for me. The first was that I was so scared of being too close to someone, since my early relational experience taught me that getting close meant getting hurt, I would unconsciously find ways to create conflict when I got too close to provide an excuse to keep myself "safe" by moving away. But this meant I consistently stood too far away to get my here and now needs met. The other problem was that SOME of our unfulfilled childhood needs are impossible to meet. The time has passed during which it can happen. So we meet someone who holds out the promise of finally loving us well enough to fill that longing only to realize that they can't (no one can) and so we move on, continuing our hopeless quest to find someone, anyone, to fill us up.
I do believe there is a way to heal from this, at least well enough that it stops interfering with our ability to live fully and get what we need now and it would include all of the great suggestions you've gotten on this thread. I wrote about it on my blog and include the link in case you might find it helpful:
How Do I Fill the Void? Erica,
Just a quick aside, you do NOT need to be actively abused to have this kind of emotional fall out. Its the disconnect of not having a loving caretaker who is attuned, attentive and meeting your needs that does the damage. This can happen for many reasons, including what you said of your family, that you had a brother who needed so much attention that you were neglected. In my own life, despite the fact that my father sexually abused me, I feel the worst damage was done by the things I didn't get rather than what was done to me.
~AG