I’ve been preparing a campaign group for one of my friends who hasn't been able to get a solid group going.

However, one of the people I asked has been... incredibly pushy and forcing me to make decisions I don’t like. All of the other players have told me they don't like his attitude, and I’ve been avoiding the problem because I haven't been able to figure out what to say... but when he told me to just up and deal with it and not argue, I’m starting to lose my mind.

Please... someone help. What do I even say? I don't want to come off as rude but I don't know how to deal with this...

Say "I'll be the GM if this person leaves the table"

All of the other players have told me they don't like his attitude

There's your answer. If you aren't interested in a confrontation (this appears to be a form of bullying in progress) then you must make clear what your conditions are for GMing. You will agree to GM if this player is absent.

1. If the rest of the players back you up, good.




2. If the rest of the players don't back you up, then this situation is
   a mess that you don't want to step into, and now is a good time to have found out that GMing for this group isn't going to work out.

Life's too short too put up with this kind of abuse in your free time when you are trying to have fun with friends. You can even use that point as a follow up if anyone asks you why.

Experience:

You might (no idea, I don't know this person, you do) get a surprise positive response from this player: sometimes, all that a bully, or a pushy person, needs is a push back to make an adjustment. I can point to two instances (back in the 1970's) when one of our DM's told a player (roughly) "Look, I don't need the grief from you. Either you get off my back or you leave." And the player changed behavior. I think (in retrospect) that in both of these cases, it helped a lot that the rest of the table supported the DM. The other cases I've seen with 'I don't need the grief' ended up in the table going forward without the problem player.

Diplomacy should always be your first start in trying to solve group issues.

Talking to the problematic player out of game and trying to solve the issue is advisable, especially since the other players have manifested their disliking of his attitude.

By your question, I can assume you have already confronted the player and he has answered with a "deal with it" kind of answer, I think it is time to take the hard route and remove the player from your game. It is never an easy thing to do but toxicity is the opposite of the purpose of the game. Players want to have fun, the DM wants to have fun and he seems to be ruining it.

If the campaign didn't start, revoke his invitation. If it has started already, explain to him that his attitude is not welcome in the sessions.

Let me be clear. You are the DM, you have final say. There are several courses of action available to you:

• You can confront him. "Hey man, you're being really pushy, forcing me into decisions and I don't appreciate it, and neither do any of the other players. A lot of your ideas aren't what I want to do with the game. Everybody's ideas will be given fair consideration but as the DM, I have final say. Please stop being so pushy. If you continue, you will be removed from the group."




• You can straight up kick him out of the group. If the rest of the group agrees with you, as you said, then they should support this decision. "Alright look man, you're being a real jerk here and we don't appreciate it. You are no longer welcome to our D&D sessions."




• You can talk with the other players and see what they want to do. "Hey guys, you all know this guy is being a bit of a bully to me and I don't know what to do. I don't want this to continue because he's making me do things with the game that I don't want to. What do you think we should do about it?"




• You can offer an ultimatum: "Alright guys, look. X is being a real jerk to me and it's driving me crazy. I hate how he treats me and I can't handle it. Either X has to change their ways/leave the group, or I'm going to leave. I'm not going to be treated like a doormat."



• 
  

  You can continue and simply ignore the suggestions that you don't like, and respond with "I don't like that idea, we aren't doing that. End of story, if you continue to pester, bug, or harass me you will be removed from the group."

  
  
  

  In my opinion, your best option is to say "What do I want? As the DM, do I think he's capable of changing his ways? Do I just want him gone? What do I want to happen?"

  
  

Put Concisely:

• Talk with him about the problem and see if he's willing to change.


• If he isn't willing to change, talk to the rest of the group about potentially removing him from the group, or finding some other solution.

A lot of the time, the problem can be solved by just talking with the person and hashing it out. Explain how you're feeling and what you'd like them to stop doing, and let them do the same. You may discover they are very willing to change, or you may discover that they are not compatible with your group and the best course of action is for them to leave.

You shouldn't have to deal with bullies in your downtime. You should be having fun, enjoying the game with friends.

Conclusion

Realize one very important thing:

YOU ARE THE GAME MASTER!!

You are the ultimate authority in this gaming group, you have final say on anything and everything. If you have the support of the rest of the group, you can pretty much do whatever you want to fix the issue you're having with this player.

Personally, I would remove him from the group, let him know he's no longer welcome to participate in your games, and be done with it.

If he's a friend of yours, your best course of action is to confront him, tell him what he's doing that's wrong, tell him why it's wrong, tell him what needs to change, and tell him what's going to happen if he doesn't change.

At the end of the day, no matter what you do, don't avoid the problem. You can't fix a problem by ignoring it and hoping it eventually goes away,

Talk with the problem-player one-on-one.

Ask them why they're being so pushy. Often times, you'll find the issue boils down to a difference in expectations for the campaign between the problem-player and the rest of the table. If this is the case, you can mold the campaign a little bit better so that everyone, that player included, can get the most out of it.

Unfortunately, this doesn't always work. Some players can get really controlling and demanding when in a game, even if that's not their normal personality out of game. It's just a matter of who they are in those environments. As others have said, at that point, all you can do is say "stop backseat DM-ing or I will have to kick you from the table, because your attitude is making it hard for me to have fun and I have reason to believe others feel the same way." (Essentially, "My table, my rules," if you want a more coarse-but-to-the-point way of saying it.) It's better to kick them while everyone is still friends than to wait for their personality to corrode relationships with the other players, but you should give them a chance to fix their problematic behaviour.