It seems like you have developed a caffeine addiction.
changes in mood, such as depressed mood or irritability, can arise since you have decreased the amount of coffee you take.
Depression may occur as part of the let down after the stimulant effects of caffeine wears off, or during the recovery period after quitting caffeine while the brain's chemistry is readjusted.
Be aware about another condition that could result from an excessive caffeine consumption, known as a adrenal exhaustion.
Its symptoms are excessive fatigue and exhaustion, non-refreshing sleep, unable to cope with stressors, feeling rundown or overwhelmed, brain fog, and many more.

Ask for caffeine addiction treatment !

It makes sense and it would explain why I'm currently experiencing mental problems.

Has anyone else experienced caffeine related mental problems? (Please share)

My wife has a caffeine allergy and just a very small amount makes her go psychotic. She becomes extremely depressed from just one cup of decaf and it lasts for 3 days with a continuing mental fog for about 2-3 weeks after. It has wrecked her life. When depressed she doesn't care about anything, so she has more caffeine because nothing matters anyhow, she continues to get worse and worse and have more and more until she reaches the suicidal stage. That took about 4 weeks this last time she fell off the wagon. This is real, she started off drinking large amounts of coffee (10 cups a day of the strong stuff) when she was in her 20's and over the next 10 years developed the allergy gradually. She has been diagnosed as bi-polar, obsessive compulsive, and more recently with some other kind of anxiety disorder, but SHE IS PERFECTLY NORMAL AND HAPPY IF SHE HAS NO CAFFEINE FOR 3 WEEKS OR LONGER. She's happy and loves life until she has that one little piece of chocolate or one decaf coffee, or that one pain killer for her headache like tylenol extra strength (they put caffeine in to keep it from making you drowsey). Within 1 hour her symptoms start to return. She immediately becomes aggressive and agumentive and really unreasonable. Thing is, when it's your mind that is being affected, you don't see the changes in yourself. So while it's clear to me what's happening (and it really irritates her when i immediately know shes had some caffeine when I get home from work, because it's like walking in on Mrs. Hyde)she doesn't think she's changed at all. Take this seriously, if you have a caffeine allergy, it will get worse with time. Also, if you go to one of these addictions groups or AA type group for support to try to stay away from it, be prepared to be laughed out on your ear, because no one believes that caffeine can do that to you. I don't believe it can, I KNOW IT FIRST HAND. See the link below and seek help while you can.

http://www.doctoryourself.com/caffeine_all ergy.html

My wife could have written Chuckles77's post, because it's my story also. I'm 38 now, and caffeine has affected me for years, although it took me a while to figure that out. I find my sensitivity to it has increased to the point that yes, one cup of decaf can affect me for days.

My symptoms, depending on how it hits me at the moment, can range from mild irritability to dizziness, restlessness, inability to lie still at night, mild social paranoia, compulsiveness, irrational thoughts, trouble concentrating, self-consciousness, etc.

Get it out of my system, and I'm fine - relaxed, happy, stable, outgoing and at peace.

I'm really embarrassed about this, because yes, people do think you're nuts...coffee...doing all this? It doesn't help that I absolutely love coffee. For the sake of my own quality of life, as well as for my wife and kids, I'm beginning to take it more seriously and I'd encourage anyone else with this problem to do likewise.