Many people are faced with the need to work on several projects at the same time. Basically, of course, this state of affairs is typical for freelancing. In such a situation, it is very important to be able to concentrate on only one project at a time.
One project at a certain time
Choose a period of time during which you can concentrate only on work and not have to be distracted by extraneous things. The period of time can be arbitrarily long, it is the working atmosphere of immersion in the process that is important. Isolate anything that might distract you in one way or another.
Preparation
Leave on the desktop only what is relevant to the project you plan to work on. Remove anything related to past or future projects. Make a list of what you need to do in the allotted time frame, gather the information you need, review the documentation, and get started.
Don't leave mistakes
It's okay to plan for more than you can physically do in the time allotted, but it's important that what you do is clear of mistakes. Take your time and do your job well. This will help you avoid having to redo all the work the next time you take on the project.
Finish
Completing a project isn't just about getting paid for it. This means that you have the opportunity to tackle the next project and not return to the finished one. It is very important that the work is done with high quality and does not require polishing from time to time - such flaws can cost you a lot of time to identify and correct them.