Just to echo what others have said, and to add my thoughts as I have some (limited) experience of garden-shed inventing.
First, your experiences are not new. This is the process that most (all?) inventions and new products have to go through - a sort of rites of passage for want of a better phrase.
Apparently WD40 is so called because it was the 40th formula for a water displacement product, after 39 failed attempts to come up with something viable.
Successful inventors need to be customer, not product, focussed by identifying thier (the customer's) need and wants, gaps, and ambitions. This is achieved best by targetted market research. If your unsolicited emails are not working then as others have said you need to put in some legwork and invest time, energy and most definitely money. ie speculate to accumulate.
Once you have a foot in the door, not only do you need to sell your ideas and concepts you need also to sell yourself - build up your CV to give yourself credibility and added value.
You should also consider that those you approach may have contractual obligations or strict procurement processes that may prevent them from direct engagement.
And most importantly: learn from your "failures" then try, try and try again.
Good luck