Depending your situation…If you realized someone isn’t a what they seemed or feelings weren’t returned you are only torturing yourself with what you can’t have. You seperate, put up boundaries and occupy yourself with other things. For most, they need to cut contact with the person for awhile, some try to maintain a friendship but it’s always met with the pain that the person will never feel the same.
Personally, I have done this to myself so many times. Attempted to remain friends with someone I am in love with or have intense feelings for, but it never worked. You get nothing, and the person is happily doing their own thing. Realize that they are one person and that there are countless possibilities in the world.
It may feel like this is the only person that matters, but they will be moving on without you. Pick yourself back up, reinvigorate yourself, treat yourself to good things, live. It wasn’t meant to be with this person, but you never know. When you’re out in the world living your life, a better person may cross your path. Don’t get hung up on someone that doesn’t feel as strongly as you do or isn’t a good person. It always results in heart break. The second someone shows you who they are, believe them. If it is not compatible with you, do not try to make it work or force it. Be honest with yourself, this person might be amazing in theory but they aren’t the one.