So, let's look at the situation in more detail. In order to learn how to convince the guys that you love them, honestly tell yourself what was the root cause of this attitude of the young man.
The first option - for a long time you denied your feelings for him, and thought that it does not suit you. If a young man at the same time tried to get your attention for a long time, he did everything possible and impossible for you, and you always rejected it, and maybe even humiliated him, then, most likely, the young man decided that he could no longer be so humiliated, and you will never love him. So now, when you realized that there are feelings, he no longer wants to believe in it, so as not to be disappointed again and not to break your heart. So now you yourself will have to make a lot of efforts to convince him of sincere feelings. Of course, we are not talking about being humiliated and turning into a slave, who is ready to fulfill each of his wishes and come running at any call. You just need to show that it is important for you, and you are ready to help him, support, share all problems and interests, and appreciate his actions towards you. In addition, you need to apologize if you know what was wrong with it, humiliated or insulted his dignity. Remember that for a man, self-assertion is extremely important and when a woman lowers it in her own eyes, and even worse, in the eyes of friends, it hurts him extremely and he can not forgive such behavior for a long time.
Therefore, you must be patient and understanding. Do not judge his suspicion and coldness. Remember that real feelings do not pass so quickly and simply. But now it's time that he can again confess them not only to himself, but to you. Try all his actions "try on yourself" and before you get angry or offended, think about how you would act in a similar situation. This will help you not to cut from the shoulder, but to understand and help him to believe that you really value his feelings and your relationships, and you also have real love for him.
A man can stop believing that you love him in another situation. For example, if he found out that you said bad things about him in the circle of friends and acquaintances about him. If you really did, then the situation is really not the best. We all know that offended by any person, we start to exaggerate everything and say those things that we really do not think. When it comes to the object of discussion, it looks truly awful. For this reason, first of all, remember for the future: it's never worth to say to someone, even in a fit of anger, unpleasant things about your loved one. Even if you trust the interlocutor and you know that he will not tell anyone, you should never give up the possibility that your words will reach the guy. This can be both a ridiculous accident, and malice. So try to keep everything to yourself, as if it were not boiling in your soul. Better say it to his face and try to figure out the situation. Even when he is angry with you, there will be a scandal and you will quarrel, it will be better than he will learn about everything from the tenth hands. Remember that gossip is distorted, passing from person to person and a guy can even say things you did not mention. But he will not believe that it was not, in fact partially what he learned is true. Try to restrain yourself and even if someone asks you, say that everything is fine and you will understand it yourself. So you will save yourself from big conflicts and mistrust on the part of the person you really love, even when you are angry with him and literally ready to kill.
If you still said something bad, it's best to admit it. The truth will still open, and a double lie will become a real blow for your young man.
Therefore, to begin with, collect the will in the fist and tell him everything that spoke then. And also immediately explain why it did exactly so. If he starts to accuse you of deceit and betrayal, do not deny your guilt. Admit that it really has been very ugly and wrong, so you regret it and understand that he has every right to be angry with you. Say that you did not want to do this, but you did and now you realize how wrong and ugly it was. Tell me that you have understood your mistake and will never repeat it. Of course, these are just words, and they are unlikely to immediately act. But still, when a person really loves, he forgives. So do not press on the guy and do not cry. Just act so that he understands - you admit guilt and deeply regret. Sooner or later he will forgive you anyway. And then, try to never again do this way and do not say too much, because the word can kill feelings faster and easier than any weapon.