No you don't need to repeat the D-Dimer test again. It has already been 25 days since you had CoViD. Your doctor is correct that your increases heart rate (tachycardia) is due to anxiety. As an ECG and ECHO can very easily detect any abnormalities of heart.
Next Steps
Try to relax yourself by meditation, deep breathing excercises and yoga. Read some good books, avoid watching too much news and WhatsApp messages. These are full of negative news and may make you anxious.