Hi there!
For sure the roleplaying scene has changed immensely over the past few years. A lot of the players have grown up with this game, and therefore roleplay styles have matured.
A lot of RPers have much more to say than just one "box" (it excedes the maximum number of characters that FH permits in one message) so they will use a symbol to tell the person they are roleplaying with that they aren't finished their roleplay post. This might be indicated with a "-c-" (continuing), ">>" or ">" or other variations. Likewise, people will indicate that their post is finished with a "-d-" (done), "<<" or "<". If you try to post whilst they are still posting, ie have not indicated that they have finished, this could be what they mean by you're going too fast. Its also referred to as post cutting.
Some RPs also rely on a post order. This not only ensures that everyone who wants to be involved in the RP is involved, but helps to maintain order so that the roleplay is easier to follow. Say we have Person1, Person2, and Person3. It would make sense to post in the order Person1>Person2>Person3, right? And when Person3 is finished, it goes back to Person1. It would be unfair if Person1 posted and then Person3 cut in before Person2 could have a chance to respond.
These are just 2 basic constructs of modern roleplaying, which occur more in literate RPs than semi-literate in my experience. The only other understanding I can infer from going too fast would be rushing the storyline of whatever your roleplaying; remember to take your time and have fun!
I guess you could say there are different "levels" of roleplaying. Many roleplay groups classify themselves as either literate or semi-literate. Literate often means lots of descriptions, detail, and over 3 sentences long. Semi-lit is less tight around these and will mostly just focus on actions and speech, and might use punctuation to signify actions such as -(action)- or *(action)*. If you want to roleplay, I suggest figuring out which of these you fall into and observing advertisements in The Grounds (people will advertise in local chat or by movie clips usually). Semi-lit RPs can be a great starting point to experiment with your style or practice your roleplaying if you want to join a literate group in the future.
Lastly, don't let a failed RP knock your confidence! We've all had frustrating experiences with other players online. If a player is annoying you or being mean to you there is always the block feature at your disposal, and you can report player to the Moderators if a player is breaking the community rules and guidelines.