Fast heart rate, very tired

Oh yes I had this and still have slightly. I've learnt to accept it plus hrt has helped this slightly. I was always trying to get a deep breath,it was the one thing I hated because it made me think about my breathing pattern which got into a bad habit. Try to ignore it. I know harder said than done. It will get easier x

Ho sweet. I have had that a few months ago very often.and it was scary I went to gp and he said it was like anxiety attack of some sort .are you maybe stressed about anything

yes yes yes..it's scary but try to relax and foucus on something else . When this happens to me first to get the heart rate (pals) I foucus on something else like call a friend play an online game. Then with breathing I breath through my nose until lungs are full then I hold breath  to  count of maybe 7 then I slowly exhale through mouth I repeat that a few times for one its relaxing and for two it gives your organs more oxygen. If you havent eaten in a while grab something with a little carbs and protien like a peanutbutter cracker hormones can and at times drops blood sugar. These are things that help me. Now as far as that drop head tiredness i either ride it through or just go take a little nap. Some months are worse than others on that terrible tirness where you can hardly hold your head up.

Have your heart checked. I was doing the same as you and I was diagnosed with Afib.