Help, I can't last a bit ..

I'm a 25 years old female, And my problem is that I can't last more than a minute ( sometimes less) to cum, and several times it happens to me when I'm sitting in a car or sometimes walking (rare) .. My boyfriend says he doesn't mind, but everytime I have to stop him during to rest my area after cumming and then continue.
Is it normal? 
Why does it happen? And is anyone out there having the same problem? 
Is there anyway to help me last longer ?