Hey, a middle aged man with crappy genetics here
Hard work is taxing because it's hard work. If it was easy, everyone would do it. Why doesn't everyone speak 5 languages? Because learning a language is hard, especially the first one. Why isn't everyone lean and muscular? Because working out and eating proper is hard. I can tell you that if you do it every day you will eventually get used to it. A great method is to force yourself to do each of these things for 10 minutes when you don't feel like it. Usually 10 minutes turns into the full hour, and you have no excuse for not committing 10 minutes, everyone has at least 10 minutes each day.
That doesn't mean that you shouldn't accept your limitations. I have muscular dystrophy, so I will never be lean and muscular. That doesn't mean I should stop eating properly and training, but it does mean I need to adjust my expectations for what my hard work will achieve.
If you have below average IQ, then you should look at what is possible for you. Below 85 you will have to make the best of your life on benefits, for instance. Otherwise you could work but it would be something that is possible with sub 100 IQ. It varies depending on where you fall, and automation might cause problems with that later in your life. Society is increasing in complexity and the IQ needed to just find a job at the bottom is constantly rising. IQ is one of the most important factors in predetermining the quality of your life. There's no need to pretend it's any different, but I am of the opinion that you need to make the best of your life despite the hand you were dealt. I just won't tell you to "Just do it!" because I know it isn't possible and I think it's far more cruel to tell someone they can do something that they can't. In the end it is something you had no choice in, you were born the way you are. I think being a person of worth is more about how you play the hand you were dealt with than the hand you were dealt itself. For instance the difference between a master poker player and an intermediate one is not so much in how they play AA, but in how they play hands like 47o since the vast majority of the hands will be "junk" rather than big pairs. Totally off-topic but I thought it was an interesting connection
Of course... be sure to actually take a few more IQ tests. Particularly a pure matrices test, but after you get out of depression. I know for a fact that depression can lower your IQ test results by a very large amount. Easily 30 points. You cannot take a test and rely on the results when depressed. Either way, if you took it before officially being depressed then I would still do a few more tests, the results can vary. Obviously don't "Train" for them because that will skew results as well, and lying to yourself won't fix the underlying problem with low IQ.
Hard work will pay off, but if you are trying to learn extremely difficult things such as astronomy then you are wasting your time with low IQ. Someone with an IQ of 120 would struggle mightily with the more difficult sciences (although still be able to succeed), and those are already in the upper echelons of IQ. I would suggest starting simple (but something you find interesting) and see where you go "I can't do this" and you will have found your limit. Chemistry is usually a good beginning, since it's basicly the "base" science on which all other science is based (as well as used in all other science). Mathematics is next, but you don't want to start with advanced calculus. Start simple and work your way up. Eventually you will find what you like and throw everything else out anyway. All I care about nowadays is statistics and probability. Everything else is a waste of my time (I am extremely lazy).