The Shaytaan uses various means and methods to ensnare people, including the exact phrases that you mentioned every time: I trust myself, I trust him, for the sake of beneficial knowledge, congratulations on the occasion of Ramadan and Eid. After that comes emotional attachment, then words of admiration, then love, and the matter may continue until it develops into things that are worse than that and more haraam.
You made a mistake when you gave your mobile number to a man who is not your mahram, and you made a mistake when you sent him a message thanking him, then a message congratulating him (on the occasion of Ramadan), and you made a mistake when you let him carry on sending messages to you.
You have two ways of ending this problem and putting an end to continuing in sin. They are:
Propose marriage to him yourself, but in a manner that will preserve your dignity and modesty.
We have discussed the way of doing that in the answer to question no. 99737
Or you can send him a message saying that you made a mistake when you allowed him to continue corresponding with you, and that you are not happy about it, and that you have decided to put an end to this correspondence.
In that case, he will either tell you frankly that he is interested in marrying you, or he will agree to put an end to the correspondence. Whatever the case, you will have achieved your purpose.
We ask Allah, may He be exalted, to help you to do all that is good.
And Allah knows best.
Reproduced from Islam QA