I don't know how old you are, but it could simply be a factor in aging. At any rate, there is wonderful advice given so far in this thread, and I hope you follow it.

I wanted to add that you should try speaking to your doctor about it. He can help be of help to you in finding out why this is happening. Until then, while I confess I don't have any experience with them, I have heard they make condoms with a mild topical numbing agent in them. That could help you to be less sensitive and last a little longer.

But please, try not to stress about it. That only makes things worse if you're anxious.

Please take care, and I wish you my best.