How are you getting on with your chinn-ups?
If you're still struggling and trying to gain strength, then I'd suggest taking things a lot easier, workout only twice a week and don't spend more than 45 mins in the gym. 1.5 hours is fine if you are trying to burn calories, but it will tire you too much to build strength quickly. Don’t worry too much about managing a full weight chinn-up for a few months, you’ll get there over time.
You can reduce you're weight by pulling-up with some of your're weight supported by you’re feet. You might find it better to start by pulling on a low bar with you’re body horizontal and feet on the floor - kind of like a reverse bench press.
As suggested, try negatives, but sometimes if you are dropping down too fast it isnt doing you much good (a negative has to be sloooooow), so starting with reduced weight would make things a little better. You said you joined a gym, so get onto the Lat Pulldown machine and you can measure you’re progress accurately until you’re pulling bodyweight - then you’re ready for chinning!
Experiment with grip position too, I personally like to use a close underhand grip (palms facing my own face) rather than a wider overhand grip - for shoulder safety and I can get a couple more reps this way. Find a comfortable bar for you’re hands as a thin bar will dig into you’re hands, but a thick one will tire out fingers too quickly. It’s amazing what a difference just a couple of mm's make for hand comfort, or wear gloves can make a huge difference. You live in a hot country, so sweaty slippy hands could be a problem, if you don’t have grippy gloves, then climbing chalk (not writing chalk as the grains are more rounded and therefore slippy) is excellent, or a piece of sponge on each palm pulled out of an old chair, bathroom sponge or even dishwashing one (with the abrasive layer peeled off) all can help lots with grip - which is the first step to chinning-up! Continuing with grip, don’t just hang on with you’re fingers, but roll you’re hands over the bar a little too far first before taking weight so that the most of the pressure is supported nearer the bottom of you’re fingers (the area where you would wear a ring), this will be a stronger grip and better place to start pulling from.
As for the number of reps dropping as you do more sets - this is fine - normal. Don’t worry about that, if you’re exercising hard then as you tire you'll manage less and less reps per set naturally. If you're holding-back with effort and not pushing yourself, then you'll manage the same number of reps per set each time - to some people that means the first few sets were a waste.
Good luck, and let us know how you are doing?