Unfortunately I can't offer much advice to the OP, actually, I'm posting to say I have a similar problem. I'm 6', 31 years old, and have been right at about 125lbs since high school. I've kind of accepted that I'm just meant to be skinny, but I went from veg to vegan a few months ago, and I started working out (both lifting and cardio) and my weight has been dropping a bit.
I definitely don't want to get any skinnier, so I'm trying really hear to consume a lot of whole grain pasta and bread, avocados, olive oil, and such, but jeez, sometimes I feel like I'm just having to stuff myself, and with few results. Argh.