Wexter, I can relate to what you are saying. It took me years to get the confidence to go off at a good pace. I think the caution is just a self-defence mechanism. You know all about going off too fast and crash/burning, so you err on the side og caution. Obviously this is of very little consequence when running the longer distances but 5k/10k you just can't afford to run a mile or 2 "warm-up".
Yes, I think HR monitor may help, but the confidence will come eventually.
I used to try and tempt myself into running a race to failure, just to see how far I got at a very aggressive pace. The thinking behind that would be I would learn more about myself and the distance/pace than I would if I just did another boring negative split.
But it takes balls to go out with that in mind, and I train hard and race easy probably because I'm a coward.