hi,thanks for query.There nothing to worry about,there could be various reasons for this like anxiety
,not having proper knowledge of the act,stress
etc.Please talk to your partner about this and spend more time in foreplay.You can practice perianal muscle exercise means tight your bottom muscles as if you are holding urine or stool.Apart from this try start and stop technique,press base of glans
when you are about to ejaculate,this will prevent ejaculation.
wishing you good health.