You tell her what you told us. The truth.

"Sadly, I need to cut back on expenses so I will be ending our sessions together. You have helped me tremendously and I'd be happy to provide a reference if you ever need one."

I would tell her you have decided to switch to group classes but will be happy to recommend her to friends looking for a personal trainer.

"I'm going to be stoping our sessions. XX date will be our last session."

Trainers have clients come and go all the time.

If you're not comfortable telling her it's an affordability issue- just tell her your schedule changed and you're unable to continue.

I have been working with a personal trainer once weekly. She is very good and has helped me tremendously. However, I can only afford once a week session and I often feel pressured to do more sessions with her. I have decided to take exercise classes through the recreations centers b/c for half the prices, I can do three classes per week. Now I feel terrible telling her this because she's self employed so I'm sure it's a tough business and also because she's been very good. Would appreciate feedback in terms of how to tackle this.