Need some advice. I’m stuck.

Hi. So I have had a problem where I tend to “freak out” on my ex as he says it. I have anxiety and especially when it comes to him changing his mind about taking another chance with me I get major panic attacks. Usually in those moments I tend to text a lot or call and try to get any response if he stops responding. Or yelling and getting more angry than I should in person.
Which the last two cases have lead to me getting blocked. I have already done the no contact rule once. Sent the letter and got a positive response. Talked a small amount went out with him one night and he wanted to get back together right away.
Then decided he wanted to focuse on himself. Then wanted to be with me again but this time he tested me. He told me the girl he was talking to asked him out and they are together now and wants me to respect that. Up till then I was doing good on the not arguing and not freaking out. But then I completely lost my composure and went into a full blown panic attack. I asked a crap ton of questions and kept pushing about stuff then he told me it was just a test and we argued. And then he stopped answering me.
The next day I tried to get him to answer. Many texts, a couple of calls, even used my old Instagram account to try to reach out to him. He blocked me on everything about 2 days ago.
I honestly don’t know what to do from here. I told myself and him that I would stop acting like that. I don’t like when I do it either, just as much if not more than him. But idk when I should try to contact him again and what to say where I already used the elephant in the room letter. Where I’m blocked i know I’ll have to do another letter but I’m afraid hell just toss it out and say yeah you’ve said that before. I just don’t know where to go from this point. Please help, I’m so afraid I’ve completely messed everything up and have absolutely no chance now…