I feel for you, I really do. Rejection is never easy. The best strategy, to preserve your own sanity (others may disagree), would be to face each rejection with a stoical nod and then move swiftly on.
It's true that occasionally (extremely occasionally) you may get a nugget of useful advice, but trying to parse that from the standard
thanks but no thanks is almost always a waste of time. There are many reasons you may be rejected, many of them being commercial reasons that have nothing to do with the quality of your writing or storytelling. Don't sweat it. Smile and move on.
This latest one said that whilst they didn’t feel able to represent me, they were sure that ‘another agency will have already or may soon be keen to represent me’.
This is nice, and certainly good for the ego, but it's still a form rejection. Pay it no mind. (I know that's tough advice, but you really can drive yourself mad with this stuff. Thick skin and a smile are the order of the day.)