我刚刚说到哪儿了？
Where was I？
What was I saying?
I just had it!

怎么说来着？
It's on the tip of my tongue...
话就在我嘴边啊！Tongue是舌头，tip是尖端，tip of my tongue很生动，表示就要想起來，但吐不出來。
How shall I put it?
我该怎么说呢？
有时候没想到怎么措辞，可以说这句，或者有时候别人问你问题，没有直接给答案，需要理一下思绪，也可以说这个。
What's the word for it...
这个怎么说来着？旁边如果有懂你的人就会帮你补上。
Now, let me see.
让我想想。讲到一半想不出下文可以说Now, let me see.　停下來想想，给自己的对话争取一下时间。

思路被打断，脑袋一片空白
Sorry I lost my train of thought.
train of thought 表示一连串的思考，比如
Please don't interrupt my train of thought when I'm writing.
我写作的时候请不要打断我的思路。
Sorry I lost my train of thought.
我刚刚一下子思路被打断了。
Sorry I spaced out there for a moment.
space out: To lose concentration or become inattentive
使间隔开；变得昏昏沉沉
加宽行距或字距
I was supposed to meet her at 9:00, but I spaced out and didn't get there till 10:30.
我本来9点就要去见她，但是我不知道咋的走了神，10点半才到。
I completely spaced out during the lecture.
我听讲座的时候整个人都飘了。
I’m such a scatter brain.
我真是脑子乱的不行

我们刚刚在说啥？
I completely forgot what I was saying?
我完全忘记我刚在说啥
What on earth was I talking about?
我到底在说啥？
I must be loosing my mind, I forgot what we were talking about.
我可能是疯了吧，我不记得我们在说啥。