I can relate to this. I did my first one last week. I did not trim which I do NOT think was my downfall. Being my first one I tried to keep as many variables out of it as possible so basically I went with the keep it simple approach. I applied rub the night before and let rest in fridge overnight. Took it out in the morning to come to room temp. Started fire in the smoker and made other preperations. Slathered the brisket with a slather recipe I found here and then added more rub.
I did not seperate the point and the flat but I think I will do this next time.
I put it on the smoker at about 240 to 250 with the point closest to the firebox (I have a snp) and the flat towards the exhaust. I did not spritz or mop for the first couple of hours. I did not flip turn or rotate. After 3 hours I would spritz everytime I needed to add wood which was once every hour to hour and a half.
Everything was going fine. Temps were climbing steadily. Remarkably fast at first. Thought I hit the plateau at about 142 but then it climbed to like 151 and sat there.
After 13 hours it never got past 153. I finally pulled it thinking it had to be done. It was. Overdone. The flavor was fantastic but the flat was dry and a little tough.
All I can determine as to what went wrong is the temp probe was stuck in a fatty portion of the point. I stuck the probe into the point and angled it sharply towards the flat thinking I stopped in the thickest portion of the flat. I think the fat insulated the probe from a proper reading.
For my first brisket it was a little disappointing. That's the thing with brisket. You get so much time and effort involved that if it dosen't come out right it's a bitter dissapointment.
So next time I will seperate the point from the flat to eliminate this issue and also have a quick read pen thermometer to back up my regular Maverick probe reading.
Also I did not foil nor did I have plans to foil. I missed the mark on my first one but not by much so I am still encouraged.
Follow the advice you will get from others on this board and you will be fine. Best of luck and hope it turns out for you but if it dosen't don't get discouraged you will learn from it. I did.