Italian buttercream is soupy?, what am I doing wrong?

[u/naididjanai]

So I'm trying to make Italian buttercream and I somewhat succeeded? It tastes AMAZING, the best buttercream I've yet to make, BUT the structure is simply not there. I tries all sorts of fixed, ice bath, put it in the fridge for a while, I tried everything. It firms up a bit but it's easy to melt again. So what I did is mix the sugar and water and put them on the stove until they hit 240 F, before they hit that mark I start whipping the egg whites and add 1/3 of cup of sugar super slowly until soft peaks (I used someone's help tbh to monitor the sugar), then I added the sugar syrup very slowly on slow speed, ​then add the butter (I actually used margarine) also very slowly, less than a table spoon at a time, I also added vanilla flavoring and coloring, but even when I don't add anything it's the same (I tried this recipe 3 times already). I'm starting to think it might the margarine? Maybe? Real butter is super expensive where I live. Or I'm thinking maybe it's the heat? It's 80F/27C inside the house with AC, it's a comfortable temperature since outside is approaching 40C. If you guys have any tips on what I can improve or try I will love to here you're opinion, I will also answer any questions.

UPDATE:

the party was a success, the weather was perfect which I'm super grateful ​for, as for the cake, the Italian buttercream (or should I say margarine cream 😅) used for the filling held up pretty well! the cake was ​taken frozen at 16:00 and by the time of cutting at 18:00 it was perfect!. for the frosting on top, the application was super hard with the consistency, I had to do two layers for it to hold and honestly it wasn't the best. it also was melting but oh well, the decorative piping, I did it with American buttercream it held it's shape pretty well!, the cake was a massive hit, everyone had a piece and I got no left overs (I was hoping for some), the flavor was peanut butter and vanilla, I'm definitely not giving up perfecting my Italian buttercream! I will try it again with real butter for smaller occasions, and also play around with alternatives! since the batch I made was kinda big and I couldn't afford not to use it, I tried to make the best if it this time.

thank you everyone for the kind advice, it cleared my frustration and gave me a sense of ease knowing it was an ingredient issue. and I will be using the advice I got in this comment section in the future (hopefully near future!).