Teenager C U M I N G problem?

hello im 14. like a typical teenager i master bate however after i PRE- c u m ( the clear liquid) even if i continue to watch porn etc my penis goes flacid and i dont c u m. after i pre c u m, which is around 5-6 minutes, a white substance can be seen on my glans of my penis ( the bell end) however this doesn't squirt out and looks like a bubbly layer. can anyone help me with this and please tell me why this happens. thank you