I've been experiencing abnormally extreme fatigue over the past few weeks, combined with difficulty concentrating and heavy heart beat while resting. ?

May be a few things: Fatigue is tricky. May be signs of many different things. Considering your age, hopefully heart trouble is unlikely but could be a sign of heart failure (particularly if you've been sick recently). Other basic things: anemia, thyroid trouble, diabetes, sleep apnea (that's a big one).

What's been going on: You may know the situations in your life that can explain these symptoms. But in any case, it might make sense to have a rule-out physical exam so you can avoid worry about your heart. And seek a good referral to a therapist to learn ways to reduce/deal with stress. If the therapist has further concern they'll refer for tests/Rx's.