Hi,
    What you are experiencing is prmature ejaculation. The commonest cause for this is usually stress, performance anxiety etc. Rarely, there can be other causes like hormonal imbalance, neurological problems etc. In most cases te cause is stress. I would recommend you to discuss this problem with your partner, try not to thinl about the erection and ejaculation. Also, having a long stimulation befor eintercourse and relaxing before sexula intercourse helps. If still there is improvement, then you need to consult your doctor for any hormonal imbalance.
Regards