Soreness and numbness in fingertips from trackpad

I've been getting sore fingertips and numbness from using my 2010 Macbook Air's trackpad. It has got progressively worse over the 18 months I've owned it. It get's too painful to touch the pad after a few hours.

I press as lightly as possible and switched to tap-to-click a few months ago and, although it hasn't got any worse, it hasn't got better either. It doesn't go away even if I don't use it for days and I'm starting to worry that it is permanent damage.

I am a part-time student so I can't simply stop using it. I am also a builder's labourer so it's not that I have soft hands.

I tend to use it on the sofa, so a mouse doesn't seem to be the answer and also partly defeats the point of having a laptop.

Anyone else have this problem?