I think I can understand a bit of what's going on here; I used to block when I imagined myself in love or someone loving me. You've probably heard this a lot, but I would take a step back and think about whether or not the guy who you thought returned your feelings was worth this distortion. You probably don't think he's that good of a person anymore, right? It's just an implication, but still. If he's not worth it, nothing should hold you back from imagining yourself with someone who
is, at least. Daydreaming is wonderful, so nothing in reality should have the power to crush it; just know not everyone's like that. Do you know how many people are in this world? Millions and millions, and the chances of you being unloved by all of them are astronomical, I would almost say impossible. So, if you can't dream still after a while, think about the ones out there who would love you, and imagine yourself with them.
My background basis for this is my first boyfriend: He liked 4 of the other girls in my friend circle, and I, in full knowledge, was stupid enough to date him. I broke up with him for being too crowding, and he immediately started dating one of my best friends. I thought everyone was going to be like that, crowding me when they could, and then forgetting me as if it was nothing, and I went into a really steep depression. I was scared to death of relationships, until I met my current boyfriend, who showed me otherwise. He is earnest in his feelings for me, I can tell, and he thinks the world of me, and I feel the same for him, and I realized: Being fifth best to a guy like that was nothing to punish myself for, which is what I had been doing by letting myself get depressed.
Please don't think of yourself as unlovable just because some of the guys in the world might not feel the same. Please let that experience go, because not being able to imagine love is a punishment you shouldn't have to inflict on yourself. You should know that you're probably a good person, because someone in the world would love you if they knew you. If you let that experience delay you, that person might never reach you, and you may miss your future. For yourself, let yourself move past it. I'm sorry my advice is so vague, but I hoped I helped you. Very best of luck, and I hope you start to feel better.