Its the suckiest of sucky things to have happen but it happens to pretty much everyone at some point. I was in a 3 year relationship with someone who didn't love me back, so I guess just wistfully lusting after them is kinda easy going. Trust me, once you've done the "thing" there is no way you're ever going to get your heart back. It propels you into hell unimagined until one day through either drugs or old age you can finally forget their name.
But in the end there is only one thing you can ever do about it. Walk away.
Because that person may date you, they may move in with you, hell mine even offered to marry me. But you can never escape the sad reality that they just don't feel the same way about you that you do about them. And that my friends is the root of all mental illness.
There is nothing quite as sad or depressing as endlessly holding up the banner of a relationship for the day when that person will finally reciprocate your feelings. If it didn't happen at the start of the relationship, trust me, it won't happen later. You'll just waste years hoping it will happen until either you get too depressed and leave or your partner finds the love of their life and does the same.
Do yourself a favour and take the short term pain of leaving now before you convince yourself life is not worth living without them. Because that's the biggest lie you will ever tell yourself. Life is worth living without them and that's exactly what you should be doing so that someone who does love you can find you. They'll never find you crying into a box of Kleenex in an empty apartment.