Any words of wisdom? Well you know, advice is not always nice. And it's never guaranteed that it's the right one for you.
You have to know yourself. From what you're telling you seem to try to understand it and I suppose it's kind of hard for any "vanilla" person to get a grasp of it in the beginning. So my basic advice is always communication, talk to your partner. This is the most important.
Be open about your fears and problems with this, talk to him about what's on your mind and what's troubling you. Eitherway you may get "an answer" that satisfies your, or not, but otherwhise you'll never know. As also, maybe you can come to an agreement or are able to see then that this is absolutely nothing for you.
Anyway regarding the fetish, easily put he simply enjoys wearing diapers, nothing else... nothing horrible. The reason behind it is another thing, they are as diverse as the people themselves who have this kind of fetish.
It's still strange right? It's nothing bad, it's probably how you may like to wear certain clothes or jewellery, which is important to you, or more simply put: Lingerie you like to wear for something more "intimate". I mean, everyone is a little bit crazy on their own, right? Your boyfriend simply has deep urge to focus on those.
I wonder, since you mentioned other kinks... you seem to be fine with them? You don't actually sound like you do enjoy them, or are you? That's important, you don't need yourself to be forced to participate if you don't like it. You could easily negotiate that you're "getting" what you like 50% of the time, if you anyway want to participate in his kinks.
However, this is kind of troubling me:
im not fond of his addiction to fetlife flirting, but do accept.
That's absolutely not okay if you are not fine with this. Period!
Of course there are couples which are fine about the idea of getting your appetite somewhere else, i.e. flirting with other people is okay. And furthermore those that life an open relationship. But if you don't want that, then you don't need to deal with this neither do you need to accept this in any way.
I wouldn't want this either to be honest. I would not accept this as a... mh good(?) relationship to be in. I would not like this very much. I mean it's a no-go for me. That's something you don't need to accept no matter how much you love him. Why anyway? It's not your fault, it's clearly his. While the fetish is noones fault, since it is none. But this is a choice, one that he clearly makes.
So if you're really not fond of this, then address it. Perhaps don't make a fuss, but mention it explicid, so there's no room for misunderstandings left.
In conclusion, you do sound a bit like your boyfriend is abusing your good nature and affection towards him. Perhaps I'm absolutely mistaken... then just ignore this. But if so, make your boyfriend remember this: Love it or leave it. Love isn't about forcing one another to do things they don't like, this is my honest advice.
Don't force yourself to do anything you do not like to do at all, love is about so much more than sex. If you want to try it's okay, but don't feel obliged.
This are just my 2 cents. If you want to see a therapist is your choice. If you really love him, but cannot go on with this then it might be worth a try, I can't tell since I don't know. It's some input and that's never that bad.
Best of luck.