There are lots of different ways to show your workload - some specific software such as Redmine or Jira mix tasks, time estimated/spend, and priorities and allow you to juggle your workload and allow you to demonstrate estimated times and so on. You can use these tools to output details of a schedule for reporting purposes - including showing how much future workload you foresee.
Alternatively there are plenty of calendar products where you can book blocks of time e.g. Google Calendar or Outlook.
The key is to check when new work comes in, look at your calendar/project management, and say "I should have free time early December, or if it's high priority speak to (your boss/person the work is being done for/project manager) and work something out". Schedule the work in, try to hit your estimated times for starting and finishing the work. If someone complains, or you are unnecessarily overworked, you can use this schedule to prove you are not able to accept new work until a future date; this gives those in charge the opportunity to address the issue by either rejecting incoming tasks, re-prioritising work, or simply hiring more people.
Ideally if you are struggling to fit work in, you need a manager or project lead to be dealing with these decisions for you. Having someone with knowledge of your workload and the ability to manage incoming work will make things far less stressful.