I'd love to know if it's possible to feel these things again. Mostly what I feel, and I think this is common for us, is anger and frustration. I can access those feelings no problem, but the rest? Nope.

Well, very seldomly anyway.
I think we have to practise feeling good. Because of our pasts we're so used to feeling bad and find it incredably hard to feel anything but negative emotion. With me it's a fear that something bad will happen if I let my guard down and relax. I bought the first-aid kit from the spartianlifecoach and use that mostly for my recovery and he says that the more you practise feeling good, the more you will feel good.
What I do these days is read humourous books, even though I rarely have any emotional engagement with them and watch things like stand-up comedy, even though I rarely laugh

I love watching funny cat videos but what often happens with them is theres some kind of dramatic moment, like a cat jumping out of the shadows. That kind of thing increases my heart rate, so I stick with fairly sedate things.
Another thing to do is focus on goals. I write, so I've started a novel. Every day that I manage to write a chapter or something I feel really good. Make yourself do the things that will lead to you accomplishing goals.
What was said by Trace, what were you like before, that's impossible for me because I've suppressed positive emotion my whole life, so there is no before.
Anyway, I think with practise we can start to feel these things. Start engaging in activities, like watching stand-up comedy, that might provoke it.