Get yourself checked out by a doctor, just to make sure you dont have a heart/blood pressure issue,, youd be surprised how many people have undiagnosed problems in this area and are totally unaware of it! (Usually a stroke or heart attack is when its diagnosed


) then learn to relax.... it takes a bit of time to learn to calm your body down, plenty of advice online for techniques, I find deep slow breaths help me, avoid coffee and sugar for a couple of hours before shooting as both will elevate your heart rate... bear in mind carbohydrates are sugar! The idea is to be as relaxed and calm as possible, so your heart rate is nice and low and your pulse isnt pounding, making you nod like the Churchill dog!! I like a firm shoulder contact but a light touch elsewhere on my rifle, so cheek and trigger hand contact is light, the lower fingers apply finger tip pressure to the grip to ensure good shoulder contact, my thumb has little contact, so I'm not gripping the gun, trigger is set as light as is safe. Cheek contact is also light, I use a piece of tape on the riser for location and a bellows on my diopter for accurate eye relief, the bellows just touching my eyebrow. Gentle , slow breaths whilst taking aim, and firing at the rest point between exhale and inhale.
A bit of practice of calming techniques (can be done anywhere and almost any time) will help over time to lessen the effect of your heart beat on your shooting