Firstly, don't be too hard on yourself. Both you and us have learnt from this episode but more importantly, you weren't hurt. I'm glad you appear to be quite sanguine about it all. Fatigue may well have played a part, but I'll suggest the design of the throttle system "bit you on the bum" because you didn't consider the possibility of a failure.
However, assuming an accurate "History of Flight" ...
Mental fatigue was the root cause...
I disagree. The root cause appears to be that engine was not "cut" once it had increased to an un-required/in-demanded power setting in excess of what was commanded/required.
Causal Factors were:
1. A throttle spring arrangement that may result in un-commanded excess power being delivered at any time.
2. A throttle (friction) arrangement which allows the above to occur.
3. Continuation of a marginal approach.
Contributory Factors:
1. Too high an approach speed.
2. Too steep approach.
3. Touchdown "deep" into the landing zone.
4. Inappropriate/insufficient contingency planning.
Generic Hazard: System Design
Specific Hazard: Throttle spring arrangement which may allow un-commanded excess power to be delivered at any time.
Generic Hazard: Procedures/Planning
Specific Hazard: Contingency planning, preparation and training for non-standard events.
Generic Hazard: Standards
Specific Hazard: Insufficient understanding and monitoring of approach performance parameters.
As the pilot and "operator" it is now up to you to manage these hazards and I'm sure you will.
Best of luck and "Happy Landings!"
PM