Best piece of DIY advise I ever got (it was not skill related) . . . however long you think it will take and however much you think it will cost, double the cost and triple the time and you will be right. I have found it to be a good rule of thumb and works out on average.
The second best thing I was told "the enemy of good is better"; we all want it to be perfect, but good enough is all you need and sometimes trying to make it better results in a lot of work for you and sometimes well you end off in a worse place. I still struggle with this one.
Painting: you are 90 percent of the way done when you star putting paint on the walls; I repainted 2/3rd of a house when I moved it, it took me about two weeks of wall preparation and taping to be ready and only a day and a half to apply paint. If you can freehand the junction between trim or ceiling and walls prep is quicker (the wife can I cannot). If you use tape, use one with edge lock and wet it before you start painting (less likely to bleed under); also score the painted edge of the tape with an exacto knife before you remove it (less likely to peel paint up with it). Take a flashlight and place it against the wall so the light runs across the wall, if you can see something under those conditions assume that you will be able to see it with color on the wall, NEVER say oh that is too small to notice . . . . Oh and seal off the area you are working in and wear a dust mask, good lord dose drywall sanding dust get everywhere.
Do not ever start a project with a hard deadline, you will end up rushing. As a corollary DO NOT RUSH; you will make mistakes, cut corners and cheap out . . . best case scenario it will bother you (even if it does no bother anyone else) forever, worst case you are setting yourself up to redo it.
The think that has taken me the longest time to acknowledge and accept is there is a time to quick, respect it. What I mean is there comes a point where you are not focused, you are tired, you got home from your job and have put in a couple hours . . . then you are prone to making mistakes (or at least I am). Took me forever to accept that no matter how little time I have been at that time or how close the project is to complete, it is time to walk away before I make a mistake that will mess a lot of things up.
Tools: Buy good tools. You do not have to pay full price, I stalk CL every day . . . and if there is something I need (and the budget allows) at a good price I jump. Them few times I have bought cheap tools think that it is a one off project that isn't that important, I have been unsatisfied with the tool and some time down the road (often years) I have a project that requires that tool and I wind up getting a better one. On the other side of the coin, there is a jig/speciality tool/insanely expensive and limited tool for every task . . . you do not need all of them.
Your tools are better and more accurate than you are: When I have bought good tools in every instance where I am using a tool as it is meant to be used, I am the limiting factor, not the tool.
Youtube and the internet in general are your friends, but a friend/family member who is competent is an invaluable resource. I've learned plenty and solved plenty of problems with the internet, but I have a family member who is very competent is many aspects of DIY who often serves as a sounding board for my projects. You would be amazed at how often a fresh perspective uncovers a truly stupid flaw in your plans.
Plan, plan, and plan again. I cannot tell you the number of times that I have come up with a plan and waited (even just slept on it) reviewed it and said out-loud "oh what a stupid I am"
Accept that you will miss something, do something wrong, curse yourself for how you did something previously, cost yourself extra time or materials and money. This is how you end up learning you will come out ahead in the end (over all of you projects).