I can understand that 10 mpm might seem slow. But it is not particularly slow, or particularly fast. Believe it or not, there are lots of people starting out that run 12 or 13 mpm. So as hard as it may seem, it's possible to slow down some. Your heart rate tends to reinforce the idea that you should slow down. 165 may be almost acceptable but the fact that it continues to climb is not desirable. 180 (VO2 max) makes it a relatively fast pace. Again, this is all relative to your current conditioning.
"Run erect, head up, slight forward lean from the ankles" (not the waist). The forward lean may be bringing your feet further under your body. You have to understand this is speculation on my part since I haven't seen you run. I usually say that you should run using a stride that feels natural to you. So shortening your stride may be unnatural enough that you are landing awkwardly, thus the feeling of htting harder. The other side of this issue is that a "natural" stride may have your foot landing a ways in front of your body, with straight leg and locked (or nearly locked) knee. This tends to transmit a lot of stress straight up the leg and also tends to generate a "braking" force since your body has to catch up to your foot.
Here is a pretty good video that talks about form and footplant. He is running barefoot, which is certainly not a requirement, but it helps to illustrate what he is talking about. Also, I don't agree with everything he says. But you can get the basics from the video, particularly the slow motion parts.