I need help diagnosing Macbook fault

I have an 2010 macbook pro, which out of the blue, started to slow down. It started out with apps taking a dogs age just to start, then it it would take up to 5 minutes just to boot and once it had booted it was unusably slow with clicks of the trackpad taking 30 secs to happen.

I assumed my hard drive was corrupted so I put it in an external case and tried to format it on my other mac and it was refusing to format, Crashing disk utility multiple times. So I replaced it with a brand new drive, within a few months the same problems started to happen again.

I would really appreciate if anyone knows of whats causing the fault and if I can fix it at all?

Thanks in advance!