I secretly have feelings for my colleague

Secretly in love with colleagues. What is he thinking?

Hello! It got me too. And really. When I saw "my" colleague for the first time, I was immediately blown away. I've never seen anything like it, but it was like love at first sight. And since then it haunts me PERMANENTLY in my head. I dream of him, I think of him when I go shopping, with friends, when I go out, on holidays, even on vacation. Just bad. But nothing happens. For one year! We got to know each other better and actually get along pretty well, but somehow we just sneak around each other. Because I definitely don't want anyone (especially HE!) To notice that I'm into him, I act more like I don't care about him, instead of the opposite. I am always really friendly, nice and in a good mood when I talk to him, but always on the buddy track. And often I'm totally ironic that sometimes he probably doesn't even know what to think of me. Pretty stupid. But he's at least as weird. Sometimes we have a really nice conversation, he asks questions or tells things that preoccupy him and then he just flits past me again as if he didn't even know me. Sometimes he looks me in the eye for ages, so that at some point I will have to look away ashamed, but then, when we are talking somewhere, he will always abruptly interrupt the conversation with me as soon as another colleague comes along to then talk to the / to entertain. Sometimes he just comes into my office to ask how I'm doing, then he ignores me for days. No idea what that is.
I know that I won't get any smarter if I don't do anything and instead just keep pretending that I don't really care, but I just don't want me to make a fool of myself if someone (especially him) finds out what is wrong with me. But then again, I just can't get it out of my head. Today I happened to see him outside (he didn't see me) and my knees went totally weak and I started shaking completely. It has happened to me a couple of times now (for example when we saw us again after a long vacation and he happily hugged me in greeting) and I have no control over it at all. Totally blatant. I have absolutely no idea what is wrong with me and what to do now. Somehow you can't forget. And I don't dare to do anything else. I feel like I was 15. However, I am already twice as old now How do you deal with such post-pubescent approaches? Does anyone know advice?
Thank you & best regards!