As a woman I can say that I often cllmax quite quickly, no matter how good the technique. And yeah, lose interest soon after ;-)
Either your foreplay is far too long, or you are pushing all the right buttons for her. (Being sporty could help - strong pelvic floor muscles help women to cllmax.)
Your best bet would be to try and find out what it is that makes her cllmax and keep it til last - that is assuming she likes marathon sessions... some women dont.
It could be that the combination of penis and pressure on the clit cause her to cllmax, if thats the case, changing position during sex & putting less pressure on the clitoris would give her less chance to build up to cllmax.
Ask her if she likes certain things - keep the ones she really likes for later - who knows, you might be doing her ultimate favourite thing all the time!
Also letting her know that you prefer to have longer, more sensual sessions could work. Get some ice cubes and other stuff involved which make it less sex and more pleasure based.