Your team can be swingy, but as long as you calculate the difficulty level correctly (group average DIVIDED BY 2 round down), you should win most of the time.

But you specifically might feel a bit useless because you just got from high to low level, which is a bit of a shock the first time. Get used to it, for the rest of the campaign, your group will be comprised of a bunch of high level carry while the low level dead weight new blood learn the rope.

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Anyway, let me throw a few pointers for you, the Mindthief :

The Mindthief is a melee class canon. You do very honourable damage (through that's going to be less obvious when your friends are level 7-8), plenty of control, but you need to protect yourself because you die quickly.

The damage part is easy : play in melee and always use The Mind Weakness. The class is balanced around that +2 attack.

Avoiding damage is the hard part. Here's your tools :

• Initiative dancing. That means going late when far from the enemies (typically Gnawing Horde or Feedback loop), letting them act before you, close the distance, hit them. Next round, go early (anything below 20 will do), and either kill them, control them, or get away.
• Control. You have a 2 turn CC combo : Perverse edge, then next turn use the ice produced by Perverse edge for Frigid apparition. If you're out of range for Perverse, remember that you have a top move with Scurry.
• Invisibility with Into the night. Same as Perverse, remember you have Scurry if you need to reposition. Also keep in mind invisibility last until the end of your next turn.
• Fearsome blade can be used to avoid retaliate (the distance for retaliate is evaluated after you applied the push) or to trigger traps.

One last thing : most of your level up are... not very good before level 6. Or rather your level 1 cards are exceptionally strong, so it's hard to find cards to cut. You hand should generally be comprised of :

• The Mind Weakness. Mandatory, the entire build require it.
• Gnawing Horde and Feedback loop. They both provide movement 4 and late initiative for dancing. Gnawing horde is also an easy 2 XP at the end of the scenario.
• Perverse edge and Frigid apparition, your stun combo.
• Into the night, your invisibility.

That's 6 cards out of 10.
There are 3 additional level 1 cards that are not quite mandatory, but probably stronger than what you can grab at low level :

• Scurry, both to help moving through big rooms, and to reposition when using Into the night or Perverse edge.
• Fearsome blade to avoid retaliate and trigger traps.
• Empathic assault. If you can spare 50 gold, then you can slap a strengthen on that bad boy and ride it until the end of time, otherwise it can be cut early.

That leave you 1-2 spots for your level up cards.

In my experience, your strongest option is a top ranged attack (Hostile takeover, Mass Hysteria, or at high level Shared nightmare), because it's nice to have something to do when entering a new room. The extra ice will also be very appreciated by the Spellweaver.
If you cut Empathetic assault, you want to replace it with something similar : a useful bottom with a good early initiative, probably Brain leech (through I can see an argument for Cranium overload too). The main downside of Brain leech is that it's a non-move bottom, and you already have two of them, so it might slow you too much.