There are many ways to manage negative energy before riding, to be more specific in my answer I would ask you what kind of energy and in what circumstances, however I will give you some general tips :
If by Energy you mean emotions then you can do the following to manage your emotions:
A) work on your physiology, on your body by changing these 4 elements:
If you want to feel sure of yourself put yourself in a physiology that represents it, if you are feeling anxious move your body and all 4 of these elements together.
If you are feeling angry or overstressed before riding then you can go for a run or smile, dance, do anything that moves your body.
B) manage your mental focus - your thoughts will determine your emotions which will determine your reality and your performance. To manage your thoughts you need to consider:
1) your inner dialogue : in what way , what are you telling to yourself and how are you talking to yourself.
2) The words you use will influence your energy, if you use words full of stress and hatred then you will feel like this, if you use words full of happiness and amusement then you will feel like this
3) When you are about to ride you can stop all of your thoUghts By using the following mantra and telling to yourself:
STOP- BREATH IN - LET'S GO AND HAVE FUN
These are a few tips to stop your thoughts if they are not useful and manage your emotions, you can also concentrate on the present and leave your problems away
There are also some other exercises of mental coaching that you can do but these should be made with the support of a coach the first time so you can understand them and put them into practice.