A few things that have helped me:
1. Ejaculate before you begin your stretching routine, then take a hot shower. Everything should be ready to stretch then.
2. If you don’t want to ejaculate, just try the shower that may help just enough to do it.
3. Look at something scary or unpleasant (for instance perhaps those surgery shows on tv)… probably undesireable long term effects but hey at least you won’t be hard for the time being (actually over time it is no longer scary/upsetting to look at).
4. Read the most boring book you can think of.
5. Run or exercise before you do your stretch routine, blood would have dispersed away from the penis.
6. Wait it out. This has happened to me before and as simple as it sounds, I just lost the urge to become hard and was able to stretch.
7. Do another PE exercise before you stretch (like your jelqs). Some people do this and report that their penis becomes smaller a short while after that initial ‘pumped’ look it gets from jelqing. Maybe then would be a good time to stretch.