This will sound cliche-ish, but everything starts and ends in the mind. It's a learned thing. Learn to somehow relax, and focus on relaxation rather than being overanxious or overexcited. Focus on the entire experience as opposed to your junior's stimulation. Also, pace yourself. When you start the act, if you feel like you're about to ejaculate, pull out and focus on stimulating her before going back in. It's a marathon; and not a sprint, so if you look at it that way, it may help to pace yourself. Book a longer session, as this may help in terms of not feeling rushed. Don't be self-centered as well. There's nothing wrong with you focusing on her pleasure even if you're the client. If you make her feel good, she'll make you feel good, and the comfort level helps with getting over any anxiety, etc.
Don't put too much pressure on yourself. Once you start the mental shift, there will be moments of premature ejaculation, before you learn to tame the beast. Don't think of it as failure; it's a learning experience, which teaches you what and how to adjust. Don't beat yourself up or stay down... Get up, dust yourself off, and get back on the saddle (it may not be the same time frame, but another day/time). You will tame the beast and close up shop every time.