There is a huge difference between wearing a costume and wearing something for a special occasion. What you would be doing is perfectly fine, you’re only wearing it on occasion, but that doesn’t make it a costume. A costume is something you wear when you are trying to look like someone else and be like someone or something that you are not. Lolita is not a costume because you aren’t trying to look like a specific character and you’re just being yourself rather than trying to be someone you aren’t. Think of it like this, a swimsuit isn’t something that most people would go around wearing every day, right? It’s meant for a special occasion, which is obviously swimming. But that doesn’t make it a costume does it?
Long story short, you do not have to wear Lolita every day and only wearing it on special occasions does not make it a costume.

They are all actually very different. Checking the Fairy Kei tag would tell you the difference between Fairy Kei and Sweet Lolita. Decora is another style (that actually isn’t all that popular anymore) that is completely separate. Decora is short for “decoration” which tells you that it’s focused on accessories things you can add to make the outfit more interesting. It’s very bold and loud and includes a lot of layering, especially of cute accessories.

Sweet Lolita:

Fairy Kei:

Decora:

As far as research goes you would start just like anyone else starting Lolita. You would do research on all the major guidelines, the anatomy of a basic coordinate, all that important stuff, and also look around and what different brands have to offer. Even if you’re not planning to buy Lolita, it’s good to look around at what’s out there just to get inspiration, so look at brands, look at magazine scans (or actual Lolita magazines if you can get them), etc. You’ll also want to do a lot of research on fabrics and materials that are often used in Lolita, good quality lace vs bad quality lace, and look around to find where you can find cute Lolita fabric, lace, and trims. Then you’ll want to look into Lolita tutorials and patterns which will definitely make your job easier. This list is a great start for that.

My biggest pet peeve is people referring to Lolita as cosplay or a costume. It’s one thing when it’s someone just out somewhere who says “I like you costume.” That’s fine, they’re probably never seen it before and I admit I probably look weird to them, and I’ll usually politely correct them with something like “It isn’t a costume, this is actually just how I dress. Thank you for the compliment though.”
But then I’ll be at a convention or something and I’ll get a “Nice lolita cosplay!” And it just annoys me to no end. You called it Lolita, so you obviously realize that I’m not trying to be a specific character, you realize that Lolita is an actual thing, so why would you think it’s a cosplay? It’s something that’s baffled me for years.

I’ve seen it before but I’ve never seen it actually look good. The styles are much too different and make it very difficult to pull off. Scene makeup tends to be too dramatic for most Lolita styles and the general color palettes tend to be quite different. I’ve seen Scene-ish hair look okay with Lolita a few times, but very rarely. The aesthetics of the two fashions are just so drastically different that it’s hard to imagine combining the looks tastefully.

Cult Party Kei is not Lolita. It has some similarities in color and certain accessories, but it does not have the same aesthetic and lacks the defined skirt shape that is necessary for Lolita.
One of the simplest ways to describe Cult Party Kei is that it’s sort of like Dolly Kei but tends to use more pastels as opposed to the jewel tones that are usually found in Dolly Kei. There’s more to it than that of course, but I’m not exactly an expert on it. I believe this is a good reference though.

It makes absolutely perfect sense and it’s actually something that I’ve said dozens of times.

I’m sitting here trying to find a nice way to say “tell the girl to screw off” but it just isn’t coming to me. But that was the first thing that came to mind. Who the hell cares what she says? Have you talked to anyone else in the Lolita community about this? Because I wouldn’t just take her word for it that the entire community doesn’t accept you. It sounds to me like a Lolita who takes herself far too seriously and has forgotten that Lolita is a fashion and doesn’t have to be a lifestyle if someone doesn’t want it to be. Lolita is all about the clothes and how you wear them, not how often you wear them.
I would just tell her “It doesn’t matter to me very much what you think about it. I have my own personal reasons for not wearing Lolita very often but I still love it and you have no right to tell me otherwise.” If she persists, go to the others in your community. Don’t try to start any drama, but I think you need someone in your community to back you up on it because it sounds to me that she is the one who doesn’t want to accept you, not the Lolita community as a whole.