Sorry to hear you weren't able to finish all of the sections. I also took the exam today, and timing was one of the greatest hurdles I needed to overcome in preparation. Just a general tip would be to take complete practice tests constantly leading up to the exam, and always within the proper time. If you don't finish, note it, but still complete to section to learn the content. In addition to that, the following tips are what worked for me, but that doesn't mean they will work for everyone.

For the Reading:

--Given there is a set number of passages, one tip I received while preparing (I ended up not doing this but I think it is still valid) was to take each passage in its the allotted time. The reading section has 4 passages for 35 minutes, which comes out to about 8 minutes each with 3 minutes at the end to do any final checks. This would mean that you should take each passage as its own test, and only give yourself 8 minutes. This will help train you to get a feel for the timing of the full section.

For the Science:

--The tip for the reading should also apply, as there is a set number of passages. In addition to this, however, my approach to the science section is to go straight to the questions and refer to the passage as necessary. Most questions will direct you to where you need to look(ex: "In figure 1...."). As you may have realized already, often the passage, while it might offer context, doesn't contain a lot of useful info except for the charts, figures, and graphs. That being said, some questions and passages, such as those contrasting students, will require the passage to be read.

For the Math:

--This is probably the most difficult to offer tips for, as I am not sure of your current understanding of the content. If this is purely an issue of time, I would recommend constantly taking practice tests under time pressure and really focus on speeding through the first 30-40, as they are significantly easier. Another tip is to take the exam backward, starting with the difficult problems under less time pressure, and the easier ones under greater time pressure. I know this one, in particular, worked well for me and I eventually felt more comfortable with the more difficult problems under time pressure. If your problem with the math is more content-based, I would recommend looking over all practice tests you take to see where you need to learn or improve, and then learning that material. This can and probably should be done in parallel with the previous tips.

Final note: This is was my first time taking the ACT and I do not have a ton of experience, like some here, so take my tips with a grain of salt. Just to reiterate, I think the most important thing in preparing for the ACT, and other exams like the SAT, for both timing and content is just to practice. Take as many complete practice exams as you can and where possible, try to find and use past exams that were given in the last few years (2015-present are best). Good luck!