Welcome!
You are getting some great advice on this thread, and i think if you follow it your situation will improve immensely. I just wanted to chime in on thermostats.
A proportional thermostat like the herpstat is the best choice. It regulates the heat by adjusting the power output going to the heating device, rather than simply turning it off and on. The drawback to these is that you are looking at a $150-200 piece of equipment to run two heat sources.
If you need to save up to get something like that them a jumpstart thermostat might be a good stop gap. It is the onn/ off type but decent quality nad fairly reliable.
https://www.amazon.com/Hydrofarm-Ger...s%2C167&sr=8-3
If that is out of range, I have also had good luck with this one
https://www.amazon.com/Century-Therm...s%2C167&sr=8-4
These control one heat source so you would need 2 to do the heat lamp and the heat mat. Another option would be to put your heat lamp on a dimmer. I like the fluker's lamps with the dimmer already installed.
Bottom line is like everyone said, you NEED to get your heat mat on a thermostat ASAP at the BARE MINIMUM.
Also, be aware that the 20gL tank will not house the snake for it's lifetime. You will eventually have to upgrade. I highly recommend looking into a PVC enclosure when that time comes. I have been very happy with mine from Boamaster, and lots of people on her swear by animal plastics enclosures. It is a LOT easier to control heat and humidity in one of these. I run mine with a radiant heat panel as my only heat source and it runs beautifully.