Syl:
My hearing aid has the same design you described - a fitted piece in the ear, and a behind-the-ear part as well. I guess the whistling is just the volume up too high.
I'm a little vague on how long it took, but it did seem like more than a month to get used to voices, etc. A month does seem a little short to me. Can you get an extension, maybe?
My directional hearing has some quirks. It is a little like a poor man's stereo, with the tweeter on the left, and the woofer on the right. I hear high pitch sounds mostly with the hearing aid, so I can't place them directionally. It must be amusing to watch me look for a bird when I hear a high pitch bird call. Some lower pitch sounds I hear only with my right ear. I am better now at many sounds, though, such as knocks on a door, or someone speaking to me when I'm not looking at them.
I guess I would say do your best to get it tuned by the audiologist, and wear it for a month, and try walking around without it now and then. If it is an improvement, you then have to decide if it is enough improvement to be worth the money. It is not likely to turn out perfect.
Steve