So I was dating this guy and he ended up telling me he needed space. He stopped being in contact with me and would ignore my texts. A month later I noticed he added someone on fb and assumed he found someone new to talk to so I messaged her warning her what he did to me and to stay away from him. Turns out it was someone he knew and she showed him the message and now he's pissed at me for doing that. I admit it was very childish, but did I just mess up any possible chance of getting back with him?
Not really, what you did wasn't just childish it was a red flag. You showed how vindictive you can be and how far you're willing to take things. Learn from this and move on. Seriously, don't pull that crap again.