Im an experienced rider that can handle all mountain pretty well, however my skill level has plat-toed.
For this part go take an upper level lesson from your local resort. Ask for a higher certified rider possible with a freestyle accredidation. They will be able to help you increase your skill level.
As far as park boards I have loved my Ride Kink board. With size 12 feet your need to look for a mid-wide to wide board. You might also check out the Custom v-rocker as I have heard great things about it. With your weight you should be able to drop down to 157ish for board length to help make spins easier but I wouldn't go to much smaller considering your height.