You just need practice man.
The people I bought the car from advised me to use the buttons from day one and the only time I have it in D is when I'm drinking beer while driving. Once you are comfortable with the buttons it's easy and also fun.
It can't do any damage changing manually.
I have put some Redline High Temp ATF in mine which has sharpened it up a little. I have found that no transmission people really want to touch it though .
On the upside, I thought it was broken but it fixed itself.