dying trackpad/mouse pointer etc functionality

I have a 7 year old white Macbook. Yes, I know it's very old. I'm impressed it's lasted as long as it has, despite having always had annoying quirks. And I'm very sad now that it's showing every sign of being not long for this world.

Earlier this year my trackpad functionality started to go. To start with, tracking and clicking started to slow down and I had to click harder to select things. As time went on tracking became very erratic and I had to almost punch the the trackpad to select anything. Eventually, one sad day, my trackpad completely cracked it. It wouldn't UNSELECT, and I was stuck with a computer I couldn't do anything with. Fortunately I had a mouse, and I was able to temporarily solve the problem by plugging in the mouse and checking 'ignore trackpad when mouse is present' in the preferences.

All was well for a few months. Then, a couple of weeks ago, I started to notice tell-tale signs again. The tracking speed was slowing down, it took a huge effort to click on anything, and even more to double-click. I understand the hardware of a trackpad failing, but when the same thing happens with a USB MOUSE? It doesn't make sense.

I have a wireless bluetooth mouse for my iMac so I decided to swap the mice and find out whether it was problem with my computer system or just a mouse problem. The USB mouse worked fine on the iMac, and at first the bluetooth mouse appeared to work fine on the macbook....until I went to plug my old mouse back in and discovered that double-clicking functionality had COMPLETELY DIED.

Since then I've been forced to use command-O to open everything, and certain features which have no alternative to double-click are completely unusable for me. The tracking and clicking has become ever increasingly erratic, and I think it's not long now till all functionality goes, as it did with the trackpad.

I know, I know, I'm due for a new computer. But I find this problem I'm having hard to explain nevertheless, and would appreciate any input on what on earth could be causing it. I doubt it's fixable but I'd still like to know.