Grondahl- When Sempai does technique on me it still feels like strength to me. Just more fluid feeling. Funny as soon as I move they stop me and say too strong. Honestly sometimes I just stand there for the whole time because I have no idea how to move without strength.
As others have said - if you don't have any good examples in your dojo of what you can feel as really using much less local muscular strength, then you may need to seek them elsewhere - they do exist
George Leonard's book "Mastery" is good on plateaus...
You are in good company in getting frustrated - see interviews with Endo sensei for example and what he went through.
Peter Ralston's books are also an excellent resource to challenging your thinking about strength.