I've fallen for a straight friend
It’s painful when you have strong feelings for someone that aren’t returned, but it’s part of life and something most people face at some point. Even if two people have compatible genders and sexualities there is no guarantee of mutual attraction. Being in love with a straight person is something that happens to most gay people at some point in their lives. It's only natural because there are more straight people in the world than gay, so it's likely that your first romantic and sexual feelings will be directed toward a heterosexual person. This means that you'll have to deal with the fact that a romantic relationship with that person isn't possible: you'll have to get over it and learn to move on.
It takes time to get over someone but you can make the process easier, and yourself happier, by getting out and enjoying time with friends and keeping yourself busy with hobbies and interests. You'll feel worse if you stay at home all day doing nothing much apart from thinking about the person you have feelings for. There's nothing wrong with having a cry and feeling bad – it’s natural - but ultimately the only option you have is to move on and work toward feeling better.
It's important to avoid making the person you like feel uncomfortable or that they've done something wrong. There’s no point laying guilt on them or trying to hurt them. Although it’s normal to want to lash out and get a reaction of some kind, treating them well means you won't damage the friendship for when you're in a better position to enjoy again.
If you have a particularly good friendship with the person you like, you might be able to tell them how you feel. Only you can decide if this is a beneficial thing to do. It can add pain and confusion to have the person you have feelings for comforting you over the situation. Still, it can be a good idea to explain what you’re feeling and that you need a little distance and some time to clear your head. That way, you don’t leave the other person feeling hurt, puzzled or worried if you choose to have less contact for a while.
Taking a step back from the friendship can be a good move. Naturally, you want to spend as much time as possible with your friend, but doing so won’t allow your feelings to lose their intensity. Watching your friend kissing and touching their boy/girlfriend won’t make you feel better either! In time you'll be strong enough to handle it but there's nothing wrong with taking a step back from the situation for a while.
It might not seem like it now but you will get over it. It just takes time, but you can ease the process by considering the points above. You aren't the only gay person on the planet and you can meet someone who is attracted to you too.