Looks like your new to PC, so welcome.
There could be many reasons behind this. You're certainly not the first person your age to experience something like this, but it's just not as common to hear about it because of the 'horny teenager' stereotype. If you're still getting hard and having wet dreams then I'd say everything's fine. If you're depressed or having other issues it can totally affect the libido. And of course, the more you worry about stuff the more you stress out and nothing kills the libido like stress.
I don't really know what to say. Is it also possible that you just don't have the right stimulation? You mentioned that when you were with your online girlfriend you still got horny from time to time. But I guess bottom line, if you're worried that it might be a functional problem then I would go to the doctor. And if you've got a therapist, talk to them. It could be psychological or physical, or it could just be a phase your body is going through. I'm 34 and I've had times in my life where I have no urge or desire at all, I don't even think about sex. Sometimes it's lasted for a couple of years. But when it comes back it's stronger than ever.