Im not sure what it is, is it Spring Grass?!- excited horses!

So I was out on the XC course with a friend the other day when my mare suddenly got excited and started galloping, there was no way of stopping her! I came off and she galloped home (a few miles away) She has never done this before and is usually quite laid back, I think it could be from a mixture of different things:

So recently we have had alot of nice weather, resulting in the grass finally growing! I don't know much about 'spring grass' but apparently it makes them lively? At the moment she has a 4 ringed gag bit, and the reins are on the larger ring, acting as a kind of snaffle? I think she needs something stronger, coudld anyone suggest anything that may slow her down if this happens again in the future?

Any advice would be kindly appreciated.

Spring grass can be quite high in sugars, which *could* create an increased energy level/hotness in a horse (it can also lead to some serious issues in regards to founder, etc).
That alone, though, should not cause what you have described above. What was happening when she "suddenly got excited" - that to me reads more like a spook/startle and bolt than a burst of sugar fueled energy. Also, since you mention you were with a friend - what was that rider/horse doing when your horse did what she did? It could be a reaction to something going on with them as well. How long have you been riding? How long have you been riding this horse in particular? What sort of training do both you and the horse have? (some of the suggestions will have a lot to do with these answers as you may/may not have the ability to act on them or a horse who would respond appropriately to them - no one wants to recommend something that would result in injury to one or both of you)
You mention that you believe it was a mixture of different things, but really only list the grass issue - what else is it that you think contributed to the situation?