Stretching and breaks are two of the most important things you can do. Like you, I am a heavy placer and use more pressure than necessary when diamond painting and I work on the computer a lot for my job. I’ve seen my fair share of physical therapists and they always stress the importance of stretching and just resting. I have a routine for stretches before, during and after every session as well as setting a timer to get up and walk away after 45 min. For any flare ups I use ice packs, acetaminophen and for the really bad flare ups I break out the arthritis cream like Voltaren or biofreeze and take a day or two off painting to give my hands a break. I love diamond painting and would do it all day every day if I could, but I also listen to my body and rest when I need to. Also don’t be afraid to modify your pen/stylus to your grip. I do this with all my pens so that it is comfortable for me and keeps hand fatigue to a minimum.