Unfortunately I don’t have a gear box on hand to take a picture of those parts at the moment, and images online are pretty hard to find.
Basically these areas circled show the trigger trolley return spring, and hidden underneath the gear wheel behind it is a small metal lever that switches the trigger block between semi and full auto.
I’ll try and dig out a gearbox to take some proper pics of the parts that @DChapo mentioned in the comments above 
is it the black part behind the trigger? so the cutoff lever?
EDIT (i took this pic from a youtube video bcs i dont have the proper tools to disassemble my blaster lol)
Well done 
Yes that’s the cut-off lever right there, which may have come loose/, out of alignment, or the trigger block spring possibly as well… which is what @DChapo was commenting about
It has that tiny little spring just visible that connects with the selector lever plate, and it’s very easy for them to become dislodged, the same with the larger trigger block spring next to it😒
found this video as well, the guy showed a scenario exactly like mine at the beginning of the video. my trolley post could be shaved off bcs its nylon. might have to ask a techy to open it up and take a pic before doing anything to it
Yes there’s plenty of videos available online for you to search and study, which is much easier than trying to explain things in text
Sorry yeah I meant the cut-off lever, not deselector.
SLR gearboxes are apparently notorious for having issues with the cut-off lever, or the early ones at least, not sure if they fixed it in some of the later units.
That and the trigger itself were a bit funky in them, the trigger was slightly shorter and weren’t always engage the trigger switch properly but would usually happen in conjuction with the cut-off lever issue.
That’s the only issue I’ve ever run into with my SLRs at least.
at first i was trying to find videos specifically for gel blasters but then i learned that gel gearboxes actually are the same thing as airsoft gearboxes lol so yeah kinda late on that one
Yes, about 95% exactly the same, so the basics of the two are pretty much the same, it’s only when you get down to the technical side of things, they are much more different than you could realise! 
Thanks for your help @DChapo
in your opinion what parts should i replace to make the whole thing last longer? bcs i literally only get 2 weeks of use before it goes busted again
Usually they lasts longer than that 
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There are JingJi specific triggers that have a longer tag that interface with the trigger switch to prevent the slipping issues, they must have the trigger sitting lower in the box than others or something.
You can replace the cut-off lever with a metal one, the stock was plastic from memory so might be prone to bending or warping.
Just had a thought they maybe the trigger spring has popped out of it’s setting off the pull is suddenly lighter. Usually if the cut-off is the issue it doesn’t reset the trigger switch on semi and it clicks when pulled but you can fix it by shooting in full auto a bit and forcing a reset.