I used to like this girl and we were just about to go out but then she started dating someone else so eventually I found another girl who I like a lot and she likes me to. But now I can't get the first girl I liked out of my mind. I don't know what to do..

Edit: srry I wasn't quite clear with what I said the first time.. I meant that I couldn't stop thinking about the first girl, and wanted to forget about her because now I see images of her in my sleep and can't get them out of my head.

Give it some time. You are not required to be in a big relationship and you might not be ready. You can date people casually (date, not sleep with!) while you get your head back together after a hurtful breakup, but you may be jumping the gun with finding a new person for a relationship so soon. Not giving yourself the time and space you need, and she won't get what she needs from you because you aren't really focused on the two of you - you have one foot still in the past. That's OK if you aren't taking someone else on a rebound ride with you.