Shrink problem

Why my penis always shrink after exercise? And how to maintain the erection?

You might be doing too much.

Stop your routine, regain your erection, carry on. The more PE you do the easier it is to get the level of erection you desire for a routine but still some days it just won’t work as well as other days.

Are you using constant heat? I always edge for 10 minutes or so after my routine. I find it promotes healing. Like Memento said you may be over doing it.

I agree with edging afterward and consider a wrap like the hair tie wrap to keep it extended immediately after. Search HTW and you will find an all day comfortable light wrap idea.