I would suggest making a list of the things that are stressing you that you can control or eliminate and a list of the things that you cannot control and eliminate. For example, the friendship, that is in your control. If it is nothing but negativity, perhaps you should put that aside. As far as your job duties and obligations, perhaps you could delegate more to subordinates or decrease your working hours IF possible. Sometimes, we chuck our days too full with stuff that we really don't need or have to do. I think a list would help you to sort all this out.
Try to find time just for YOU in all this even if it is just 15 minutes/day. "Me" time is very important.
The above poster offered some great tips.
With my experience regarding stress, it can manifest itself in many ways. Forgetfulness, anxiety, anger, feeling sleepy... all of those things were my frequent symptoms.
The good news is, there are a lot of techniques out there to help deal with stress. One easy one is more exercise. A simple walk with some intent of motoring through what ever is bothering you, or simply forgetting what is bothering you and notice all of the small things around you.
I've personally developed a technique that is kind of like meditation and deep breathing exercises to clear the mind, and then see whatever is stressing me out getting smaller and smaller until it no longer exists.
There are numerous self help books out there as well.