How To Cure Extreme Tiredness After Undergoing Stents For Heart Attack?

I had a heart attack 6 months ago and ended up with 2 stents. I now feel very well and look after myself but the tiredness is horrible. I work 4 days per week, Practice Manager at a Medical clinic, and on the weekends if I go out at all I will be exhausted the next day. Definitely not able to go out during the week. I find my energies take me to about 3p.m each day and hopeless after that. I am not depressed and on the usual medications after a heart event and have been told they can make me tired. Any ideas for relief. I am 64 years old and worked all my life and now thinking maybe retiring will be the only option