just take it slow man! i never had to stretch anything. just wait a few months or even half a year! than it just fits! i got from 2.4 mm to 3.2 to 4.0 to 5.0 mm in two years. now at 6.0 mm, i will not go further.

I stretched my piercings myself. My PA I stretched from 12g to 0g. My suggestions are -
1. Take it slowly, always leave at least 3 months between each stretch and only stretch up 1 size at a time.
2.Use a stretching taper and plenty of lube (I used KY) and again take it slowly. It used to take me between 30 minutes to 1 hour to stretch. Push the taper through until it starts to hurt, then leave it for 5 to 10 minute in place, let the piercing hole get used to it, then push it through a bit more until it hurts and just repeat until the taper goes all the way through.
3. do it in a very warm bath. The hot water helps to relax the skin and tissue, this helps the taper go through easier.