That is really tough for you. I have a similar problem, but it's just any time I see another client I get freaked out! Also I used to see my T at her house where I never saw another soul, she must have staggered the appointments so we would never cross paths. Now at her new office the sessions are all in a row with no time in between. The client before me today was late finishing and I was anxious and furious. I wish they could space their sessions. It just seems like common courtesy and avoids so many extra feelings!
Please try not to be concerned that she's 'prettier.' Maybe your T would disagree or maybe he finds you more interesting. Your T will be focused wholly on you during your session - that's what you pay him for. Maybe you can think of a way to soothe your feelings so they don't encroach on your time with him. After all, you deserve to be there just as much as any other client and deserve his attention as much as your own.