I don’t know if you’re still here or not, but nobody has replied to you so if you are in the thread, here’s my advice as a demomain.
You are the demoman, this means a lot of things, but it means that, most importantly, you’re good at doing splash damage to clusters of enemies. If you see someone fighting a teammate, you can lay stickies at the ground around them to force their movement options and either rig the fight or blow them up.
What it does not make you good at, however, is 1-v-1 fights. Demoman’s DM game is one of the worst, which is especially bad as Scouts, your counter, have one of the best. Basically, if a good scout puts you on his shitlist, unless you get lucky you’re going back to spawn. So yes, sticking with your team is a good idea, but what if you’re on a pub and your team is rather incompetent? Well don’t fear, I have advice for that.
First, GL + Chargin Targe may sound cheap, but it’s important training wheels. This shit will teach you to use the pipes, and this is important as the pipes are one of Demo’s best DM tools. Your second big advantage is that you break the rules of TF2. Every class in the game except for Demo and Sniper are close-range fighters. Even Soldier, who can preform at mid-ranges, doesn’t do his optimal damage unless close. The demoman has no damage falloff on pipes or stickies, so this allows you to fight very well at mid-range. Keep in mind that your explosions harm you and stickies have arming time, so close range is not good for you.
Manage to out-skill people and they’ll be afraid of you. If they’re afraid of you, instinct will cause them to keep a distance from you, where they’re weaker and you’re stronger. Use this to your advantage and you’ll learn why people cry that the demo is OP.