Hanneke, this is a frame of mind that is familiar to me (though not at this moment). I think cycling can be a very 'up-and-down' sport emotionally.
At the amateur racing level I think it is more demanding than running and certainly more dangerous. And more time-consuming.
A couple of 'offs' or a bit of overtraining or a really bad ride where the legs just aren't there, or a combination of all three, can really dent one's motivation - more so than running I think, cos it's harder to find an excuse not to go out for a run.