applying paragraph styles

This is not the correct approach. It only works by accident.

mysel.paragraphs[0].insertionPoints are all the insertion points in the current paragraph. If there are 100 characters, then there are 101 insertPoints (on each side of each character). insertionPoint[0] is to the left of the first characer, [1] is to the right of the first character, [2] is to the right of the 2nd character, etc., up to [101] to the right of the last character.

insertionPoints[-1] refers to the last insertionPoint, [101], which is actually after the paragraph, per se, so formats the next paragraph.

insertionPoints[-2] refers to the 2nd-to-last insertionPoint, [100], which is the the left of the last character.

I'm not sure what the best way to deal with this is. Perhaps:

mySel=app.selection[0];
p = mySel.paragraphs[0]; p.appliedParagraphStyle="myPstyle1";
p = p.insertionPoints[-1].paragraphs[0]; p.appliedParagraphStyle="myPstyle2";
p = p.insertionPoints[-1].paragraphs[0]; p.appliedParagraphStyle="myPstyle3";
...

I suppose the exact method depends on what you're trying to do.

Use previousItem on your current paragraph:

http://jongware.mit.edu/idcsjs5.5/pc_Paragraphs.html#previousItem