Kneel down or try to sit down/let your legs go, I find sitting works but some people do it simply leaning forward. If you feel intense blood build up (pain) in your head, there probably isn't enough pressure on the noose.
Yes, you can simply stand up to release to abort. If you have a good ligature and knot you will still have to pull at that as well when you stand up to loosen it.
The chances you follow through to completion the first time attempting are most likely very low, it may give you the realization that your not ready yet. You'll probably have to play with it a bit to get it right unless your extremely determined and just go for it.