Its one thing to know how to fix specific things. Like a DVD player, desktop computer, etc. But, you can't possibly learn and know how to fix everything. The human brain cannot hold that much information. You most likely will find yourself in a problem you didn't learn how to fix.
What now? Use you problem solving skills.
1) First and foremost identify the problem. (Well, first be emotionally ready to confront the problem. In other words, don't freak out, don't be scared.) Find the true source. What ignites the situation into a problem? Look as reasonably deep into the situation as you can. Remember, there could be multiple problems to fix in order for you to get to your goal.

2) List problem(s) and goal(s). Decide how the problem blocks you from your goal. If your going to fixate on something, fixate on the goal. The problem you chose my not be the source. You'll end up going down a rabbit hole, solving a problem that doesn't achieve your goal or that involves more than needed. Keep the goal in view, it will highlight the real problem.
3) Come up with as many solutions as you can. A good list can make your job easy: Try to keep each idea different, write them down if that helps. Use what's around you, try not to add more complexity to the situation. Take your time, be patient, do it right the first time.

4) Involve the people around you. Without losing organization, involve everyone. Make teams, assign jobs if you want. Communication is a big part of a relationship, don't let a problem ruin that.
5) Pick your best idea, imagine how it will play out. Imaging all the little details. Physics and math knowledge is important. You'll need an understanding of psychology for personal and relationship problems. Use self-education to understand how things work, not how to fix them. Don't learn, 1+1=2. Know that taking two or more values, consolidating them and giving them a new summed value is the process of addition. That way you can add any values, while remembering less algorithms. You'll be able to apply more knowledge to more situations.
6) Implement the best idea and see what happens. You want to try the least damaging idea first. Try things that can be put back to normal or reset. Then move on to the more risky stuff.
7)Did it work?
- No? Why not? Something is not working. Look at the problem from a different angle. Make sure you have involved every aspect. Now start back at 1.
- Yes? You're done, problem solved. Repeat if/when necessary