How can I become more flexible so that I can perform fellatio on my self?

Have a few ribs removed with an elective surgery.

It requires being thin and having a very flexible spine, plus some usable penis length helps, too.  If you're thick or overweight you have too much flab in the way that prevents you from curling up to suck yourself, so any gut would have to go.  And being young and limber helps the spinal flexibility.  Practice with stretching out and curling up will help push your limits.  But if you're getting older and your spine is starting to stiffen, you may not be able to limber up the flexibility enough to ever suck yourself.  And being older, you also run the risk of rupturing a disk in your spine if you try to curl up and push yourself beyond the limits of your flexibility.  Be extremely careful if you try.  Also, I've read in the past that yoga can help since it really exercises the body and improves the flexibility.

If all else fails, just 69 with a male friend.  It'll almost be like performing on yourself since you'll be feeling what he's doing to you at the same time you're doing it to him.

It’s impossible. Your rib cage prevents you from bending that way. You’ll have to find someone else to do it for you just like everyone else. 

it's all in your djukus. go practice your djukus.