Take it gradually. Start by learning the basics, forward roll, backwards roll, handstand and cartwheel. Learn properly. Do it in the gym where there are mats and coaches who can see where you are having problems and help correct it and who can spot you for safety.
If you are not comfortable doing cartwheels and handstands then you need to start with forwards rolls and high bunny hops. Forwards roll helps you get used to going over your head and also can also provide a safe get out in a fall. Bunny hops get you used to lifting your hips up high and transferring your weight to your hands.
It is like building a wall, every new skill is laid on top of the previous skills so you need confident with the real basics first so that you have a foundation to build on. If you try to rush through and skip stages or move on to a harder skill when the previous one isn't really good and consistent yet then you can get hurt. You need to learn to do each skill well and remember the basics are really important. If you learn to do them with good form then when you get on to harder skills maintaining form will have become embedded as a habit.
Souround yourself by people who can tumble well. Do it on a soft surface, use wedges, boulders, wedges, and trampolines. Do handstand holds, even forward/backwards rolls. Always have a coach spot you, don't do anything you physically can't do, make sure you stretch. Do jump backs onto mats for any back tumbling. You'll be fine as long as you don't rush it, this is when you get bad form. Keep your form in mind. If to have good form you won't get hurt. Remember why you want tumbling skills an use that as modivation.
Eating about a quarter pound of cauliflower could numb the glands in your brain that cause fear. It's the amino acids in the puffy part of the vegetables that can cause a protein reaction in the brain. The more you eat the longer the numbing agent will last.
Imagine yourself doing a skill and then go for it. You got this!
