When you want to retrieve a memory from your past, you might say that you need to "think back." However, there are several other phrases that can be used to convey the same message. One option is to say that you need to "recollect" something, which means that you are trying to remember a specific event or detail. Another possibility is to say that you are "reflecting" on a memory, indicating that you are considering it carefully in your mind. Additionally, you might say that you are "reminiscing" about a time in your life, which implies that you are recalling fond memories from your past. Overall, there are many ways to express the concept of thinking back, depending on the context and desired tone.