Hi Astro,
I don't know if I can help, but I'm going to try. I am someone who was always interested in kink, but never diapers. Then I met and married FluffyTheGryphon, and learned how to embrace being a carer for someone who identifies as ABDL. Maybe this will help.
When I met Fluffy, he talked to me about diapers before we married. That allowed me time to let him know I was open to easing my way into having diapers in my life. We took it slowly and discussed why diapers mattered to him. Now, diapers have an almost daily place in our lives, and I enjoy the trust and intimacy we share as a result.
Telling anyone you are an ABDL involves a great deal of trust and you have done the right thing by initiating those discussions. Considering he gave you an ultimatum, it sounds like the diapers are a deal-breaker for him, which makes me sad.
These are questions you certainly don't have to answer, but for you to consider: How long have you two been together and communicating about this? In what way has he tried to understand? Has he done any reading (of good, reputable books) on the topic of ADBL?
I don't know if reading about ADBL would help your boyfriend at all, but he could try a couple of books, if he's willing. There are two very good ones currently available on Amazon in Kindle Unlimited that I recommend to anyone who seeks to understand their ABDL partner. They might help ease his mind in seeing that there's nothing weird about this. Also, have you told him why this is something you enjoy and why it is important to you?
It's an unfortunate truth that not everyone is accepting or understanding when it comes to ABDLs. Does he understand that the positive feelings that come from wearing diapers are not something you can simply switch off? Does he also know the diapers not a replacement for his love, affection, and cuddles?
Do you want diapers to be a part of your relationship or something that you can do on your own time, so long as he doesn't mind?
I know that's a lot to examine and consider and, like I said, I'm not looking for you to answer here. I'm sharing thoughts and asking questions in hopes that you will both be able to have a productive dialogue and move forward together.
As far as therapy, I wouldn't consider being ABDL an "issue" that you need to go to a therapist for. A therapist's job is to help you connect with and love yourself, which quite possibly includes loving the fact that you are ABDL. Of course, I don't know you, so I can't speak very specifically to your situation. But if you feel in any way that therapy would be helpful, it's important to go to someone who won't judge and try to "fix" you, but reassure and listen to you. Good luck!