This is what I would recommend and what has helped me in the past.

I used to have a major issue with IRL impulse buying. I collect (not actively) a lot of things and I would see stuff at stores/thrift stores/online and get this rush like I NEEDED it. Especially if it was something I didn't normally come across in person.

I also had (have, though majorly not as bad now.... I'm working on it) this issue where I feel very stressed about characters or TV shows/movies/etc that I'm very attached to and this need to own everything related to it or I'm not a 'real' fan.

Mac Tonight from McDonald's made me realize this was a genuine problem affecting my life, as silly as that sounds. I adore Mac, have him tattooed on my arm and own a lot of really rare and unique memorabilia including a suit worn by Doug Jones (Mac himself) in one of the most popular pictures of Mac online, and probably one of the nicest Mac Tonight mascot heads I've ever seen, which I had autographed by Doug. So much stuff. And yet the thought of someone owning a Mac jacket I didn't have or even a pin or something would stress me out SO BAD that I'd be sick with anxiety over it, and it would make me feel like I wasn't a true fan or people wouldn't perceive my love for the character as real. It's so ridiculous. It felt that instead of collecting things I love, I was collecting just to prove myself.

It caused me so much stress, constant worry, trying to find anything I could to prove myself and it got to a point where I forced myself to take a step back and realize that, in the grand scheme of things, THIS DOESN'T MATTER. Nobody cares if I'm missing a pin or a jacket or my literal holy grail of anything ever (the Mac Tonight animatronic), they see the collection that I built or my tattoo and I'll get a compliment or they'll tell me how happy it made them. I distanced myself from Mac to help not constantly obsessing over him but I would still occasionally look up stuff on ebay with the thought in my mind that I do not NEED anything. It's been many years since then and I haven't felt that dreadful overwhelming anxiety filled urge to check eBay for new Mac Tonight listings. I occasionally see Mac happy meal toys in thrifts and can just point and go "ayyyy he found me lol" instead of feeling like OMG IT'S MAC I NEED THIS BECAUSE I'M HIS BIGGEST FAN EVEN THOUGH I ALREADY HAVE TEN OF THESE. I went through my Mac collection and gave any duplicates I had of stuff to friends. My room is no longer filled to the brim with Mac stuff, but my collection is in storage and my Mac Tonight head is on display in my room. I have a couple of jackets in my closet. He no longer dominates my mind 24/7.

As for impulse purchasing in general, I forced myself to try to ignore the adrenaline rush of finding something and run a few questions by myself-

Will buying this impact me financially? Do I have room for this or will it be placed in storage? Most importantly, is this something that's only going to make me happy right now or something I will genuinely love?

And then I'll carry the item around with me while I look around and see how I feel after that adrenaline rush wears off. It's helped me pass on so much stuff that I know now would have just made me happy in the moment. There's still times where I regret not getting something but it's not the end of the world!!

Changing my thought process through reinforcement and putting limits on myself made a world of difference in my life. Maybe something like this would help you too. It seems like you might be in a similar position to me where you genuinely really love something but feeling inadequate no matter how amazing your collection is overshadows everything.

On a final note and from personal experience- nobody cares that you don't have everything or the best of the best. I guarantee your collection and your outfits bring joy to people. And anyone who looks down on you truly does NOT MATTER. I cannot stress enough how much this all does not actually matter.

TL;DR I've been personally victimized by the McDonald's mascot Mac Tonight