So, how do I get out of the vicious circle?
Stop doing the thing that's causing it. You diagnosed this yourself:
It's probably the worst issue I have as a Game Master, I think of a Game, I write a campaign plot for it, End, Beggining and Middle, get Hyped, Hype my players, and after 2 months I want the story to end, and it's usually too late to make changes to the GMing style I use by then, so I just dissapear some weeks and invent I have stuff to do, cancell the game, and run another one.
- You find a game, like 4e, and a beginning-to-end plot for it.
- You play it for a couple of months.
- You cease to be interested.
- You find a new and shiny thing to be excited about — which is fine, there's lots of new and shiny things it's pretty reasonable to be excited about. This might be another game like D&D 5e, or a new kind of story.
- You lie to your players (that is what you're doing) and stop the current game.
- Return to step #1, and repeat.
It's absolutely no surprise one of your players seems concerned - she is concerned, and has good reason to be. It sounds like you've done this at least a couple of times, and your players probably realise by now what's going on - including that you're not being honest with them. You are clearly not having trouble running games in general, though you have stuff to do sometimes. That's fine! Strangely enough, though, whenever you're available to GM again, you aren't interested in continuing the previous thing.
It doesn't take much for someone to see through to what's really going on. They have probably recognised you're just becoming disinterested in stories. Now it has reached the point where you're beginning to hear concerns about it.
That said, it takes some courage to confront this stuff and ask people about it, so kudos to you for having that courage and doing that.
So, how do you break your cycle and stick with one game and enjoy it?
There are always going to be new and shiny things. You are totally justified in getting excited about them. The problem is not that there are new and shiny things, it's that you're letting yourself cancel your games all the time prematurely.
There's a few options to break this problem:
1. Run shorter campaigns
Run shorter stories that only last a few weeks. Have player buy-in that they are going to be short campaigns and that the story will not necessarily continue. (Of course, nothing's stopping you from continuing the story a few months later.)
This lets you switch every now and then harmlessly.
2. Have some discipline and stick to your campaign even if there are new and shiny things
That's it! That's the key to finishing a campaign: it means having the discipline to keep going. So do that and keep going.
In 2002, Richard Machowicz, a Navy SEAL, was interviewed by Larry Connors. You can read that interview here. In that interview, he talks a lot about courage and how people are able to see things through to the end. That's probably relevant here. Part of what he said was that to complete a mission you set yourself, you have to have courage on three distinct levels:
- You have to have the courage to make the choice to begin.
- You have to have the courage to keep going.
- You have to have the courage to finish.
You are fine with #1. You are letting yourself down with #2. You need to have the discipline to keep going. Push through the bits where you're disinterested. Push toward keeping yourself interested.
If you're having trouble staying interested, I suggest you stop writing stories that have a beginning, middle and end. Design stories that have a beginning, and then put your players in that beginning, and discover the story with them. You may be bored because you already know the entire story, so whilst for the players this is all new and exciting, you already had the new and exciting experience whilst you were writing the story, and now you're just waiting for all the stuff you planned to happen, which is boring. Not knowing the middle and end, on the other hand, means you're discovering things just as your players do, and exciting stuff is happening for you as well.
BESW and I run games like this: generally, at the beginning, there is a situation at a tipping point where the status quo is about to be disrupted (e.g. two factions are reaching the peak of tension and about to break out into war, and a third faction is rising in the shadows), and the players are positioned as the perfect X factor, with the power and autonomy to influence how events pan out. And then we let them plan out, and give the players and their characters that autonomy. We loosely plan only a session in advance, and we have people with their own individual goals (and not scripts) who we can improvise easily enough, because the players will probably surprise us by doing nothing we expect, and ruin any plans we might have in the process. And then we have fun discovering the story with them.
Try that out.
3. Have someone else GM
Really. If you think your GMing style is a major problem, have someone else be the GM who is willing to run this in a way you're not. It's pretty simple. Then join in and play with your group.
Also an option: open up and be honest with your players.
Tell them what's going on. Tell them you're having trouble staying interested in the games you're running. Tell them you are getting excited about new stuff. Tell them you'd like to be able to run these stories through to completion, and that you're having difficulty doing so. Admit to them you were lying about having things to do because you weren't sure how better to handle the situation.
This might take a little bit of courage to own up to, but it's going to have a lot of benefit.
This will put this issue out in the open, and open it up for people to talk about it. It will likely only confirm what they already suspect but do not feel they can talk about.
Your players will then actually be able to talk about this openly with you, without so much tension and awkwardness being there. Some of them, possibly the girl who was concerned about the current story, will be able to talk to you about possibilities around revisiting that previous story or character she really liked and wants to see more of.
Some of them may be upset. Some of this upset will be upset they already have there because you keep cancelling games, but have not been able to express because this issue has not been open. Be prepared for that, accept it, and be compassionate toward them. If they're your friends, they're generally going to be on your side.
It will also relieve you of the heavy burden of having to deal with all of this all on your own and having to keep it all secret and everything. It will let you actually talk to them about it, get their help and support, and as a group you'll be able to work out how to go ahead from here, among the options in other answers and this one.
You're only human, so have some compassion for yourself, and let the people who care about you help you through this.