if I let my car sit for a long period of time when I get in it it will be dead

I would suspect a battery issue. But if you have had it tested, problem maybe elsewhere. The battery may have a small draw that over time will deplete the battery. Look for easy to find problems, glove box light, other interior lights, new addition to the sound system that stays on all the time. Some problems are easy to find, some require a technician to accurately diagnose. Good luck, and give us a shout if we can be of any help.