I still love my ex ex and don't know what to do?

I still love my ex ex who has a girlfriend. We slept together last week and it has left me confused and low.

I still have feelings for my ex from 4 years ago. This is my ex ex and not my ex. I have a child with my ex boyfriend who i split with a year ago. we split up because of a lack of feelings. Anyway my ex ex got a girlfriend as soon as i got with my ex, i rejected my ex ex when he said he wanted to be with me so he then went and got a girlfriend whom he is still with now 3 years later. We talk on and off and i can tell that they are happy together and he loves her but he says he wants to see me a lot of the time. Last week he eventually came to see me and we had sex. He left soon after because his girlfriend kept ringing him and this made me feeling awful. This was a week ago and since then I've slipped into a depression. I feel so lonely and unhappy that i still love him and he is with her and i have realised that he will never leave her for me. He says they argue and gets annoyed that she always wants to see him when we've planned to ( we have cancelled plans countless times) i can't believe i sill have feelings for someone from so long ago who isn't even my ex or the farther of my child.

It seems mental, maybe i am crazy? Do i need to give up and let go of something that is never going to happen? Or does the fact i still have feelings mean something? Does he still have feelings for me or is he using me for sex?

Please help, i can't go on this unhappy, sometimes i wish i could completely erase him from my mind, it feels like i am doomed for the rest of my life as i am constantly thinking of him and longing for him. I go on his and her fb all the time and cry at their pictures together. I am a 24 year old professional woman and mother of a 2 year old. I am together in all other aspects of my life but these feelings i have for him ruin me! My happiness depends if he has spoken to me or not. I can't go on like this.
I still love my ex ex and don't know what to do?
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