Today my plan was to do some threshold efforts, but immediately at the beginning of the ride my HR was super high and I felt powerless. I tried to go a bit harder on a little climb for one minute; I was just gasping air and struggled to hold 340-350w! (my threshold is 350-360w. well should be!) I tried to do also an short sprint, and maxed out 1201w (2 weeks earlier maxed 1430w).
So I ended up doing one shitty hour. 187w avg./ 156bpm avg. (about 30bpm higer compared when i feel fit!)
Why didn't you stop the training? Your goals where too high and unreachable. So why train a 'shitty hour'? It's pointless, has no effect and brings only extra tiredness.
So what could this possibly be? Becoming ill? I don't feel like it though. My appetite is normal and i sleep well. My motivation for training is quite low at the moment though...
The last sentence is one of the signs of overtraining.
Take a full week (at least) of training (just do recovery or base), feel if you're getting fitter and after a week try a short but hard training (like 5 or 6 max intervals, with about 2 min recovery). Short training is about one hour. Check your data and then take your conclusions.