Therapy can be in emotionally draining.
This is especially so, when you begin to open up about stressful and traumatic experiences and memories. We often bury painful experiences and avoid talking about them, because we don't want to relive an awful and devastating past.
Within a safe environment it is actually helpful to process these feelings and emotions as over time it will become easier to manage those emotions, and the responses to those emotions. You may even enjoy a feeling of relief once the burden has been lifted.