I can't concentrate, my reaction and thought processing is slow. When do I get !

I can't concentrate, my reaction and thought processing is slow. When do I get concerned?

I am not right. I know this, because when I speak I constantly am stumbling over words, or losing train of thought. I can't focus. I function in a productive manner, so these issues aren't horrible, but it is clear that for the last few months, my brain is not working like it used to.
What concerns me the most is that I notice a difference in my driving. My reaction time is slow, but I think it is because my focus is not there. I actually avoided what was almost a terrible accident the other day, so I don't think this is a constant issue, but its like I see a red light and my brain has to process through the idea that I have to stop.
I am probably being overly aware of myself, but I was just curious if I should worry that something is physically wrong with me.
For a long time, I chalked it up to stress. I just went through a stressful, painful breakup. I also work ALOT, so I figured thats what is going on, it doesnt seem to be getting better. I have good days and bad days

3 weeks ago
Please don't tell me to go to the doctor. Give me insight if you have some...if not, then no need to say go to the doctor.

3 weeks ago
I'm 28 and female.

I'm going to assume that you're not losing your reaction time due to age, because if you were of that age, the answer would be obvious and you wouldn't be asking us.

Depression can present with difficulty concentrating and slowed cognitive functioning. Given the painful breakup and the stress at work, that would be my first guess. You're probably not interested in medications if you're not interested in doctors, but there are other treatments/therapies. I'm just throwing this out there as my opinion. I may be biased toward a mood disorder since I study them, but to me it fits.