Firstly you need to break away from those dating sites. Seriously, there is no justification in what you are doing there. You risk doing serious damage to your marriage.
It does sound as if your wife is suffering from depression try & talk to her about taking the anti-depressants, they really are important. They may take a few months to kick in, but she will start to see that light at the end of the tunnel again.
Communication is important here, you need to listen to what she is saying. Somewhere in her words are the answers as to why she's feeling the way she is. Don't try & fix her or judge her just listen to what she says & take things from there.
Take time out for yourselves as a couple, get someone to watch the little one & take her away for the weekend. Rent a little cottage by the sea. some places in the uk have lovely coastal walks. The sea air & the change of scenery will do wonders for her. I always believe if the love is still there, then it's worth fighting for.
Good luck.