I'm just a regular slow guy, I'm sure an experience racer can actually help, but for me, getting a powermeter and doing an honest all out 20 minute effort to get an idea of my ability let me know that I can push myself much harder than I was (especially for a time trial) let me know that I can push very hard (for me) and keep it up for the whole time. Heavy breathing/heart rate wasn't a good indicator for me of how much power I'm putting down because if you exhausted your breathing/hr stay up even though your not putting much power out.
I think a powermeter helped me find a point where I can tell my legs to shut up and just keep pushing because I know I can do it, even if my muscles are saying I can't.