Hi there. Were you in good health when all this started? Do you take any medications for any reason? Any supplements?
The normal CT and MRI are very reassuring vis a vis a progressive neurological disorder. Also your age would mitigate against this heavily. In the abscence of documented organic pathology, one would have to postulate that you are experiencing the very early stages of a systemic inflammatory disorder (rare) or, more likely, the effects of cumulative stress, anxiety, poor sleep, fatigue, caffeine/supplement use, etc (very common these days).
Yes, given what you have said here, you can rule out brain tumors. Certain immune disorders like fibromyalgia, Lyme disease, syphilis, amyloidosis, mononeuritis multiplex, etc can have variable neurological signs and symptoms associated with them, though this is less likely.
I do think focusing on your stress would be the most promising approach. As amitriptyline can be sedating, you might want to try something else, perhaps an anti-depressant/anti-anxiety medication such as lexapro or celexa. A mental health professional can help you to get this under control.
Sertraline belongs to a class of medications known as SSRIs. They are generally very effective for anxiety...and the depression that often goes along with it. Sounds like a reasonable choice. Just be aware, it can take several weeks or more to notice improvement. As for your memory, this should improve along with the anxiety, as anxiety is a form of distraction that impairs the acquisition of information and its retrieval from the memory centers. In the meantime, carry a small notebook and pen with you and write down important information, so that you do not forget. Using a cellphone can also be effective, but sometimes the act of taking out a notebook and writing actually improves retention.