if you say "I'm in over my head" it means that you have too much to do, you accepted too many tasks and now you cannot finish them. I think what you probably mean is that you are now very upset or frustrated that you did not go to Hakone because you could have made it there since it stopped raining. One informal phrases you could use to express that is "Now I'm beside myself." Or you could just say "Now I'm upset" or something similar. Hope that helps!

I think 'however' works better in the last sentence so, to avoid using it twice, I have replaced it with 'but' in the earlier one.

Well done!