Reddit reviews How to be a Productivity Ninja 2019 UPDATED EDITION: Worry Less, Achieve More and Love What You Do

It sounds like you're in a position of an ad hoc decision maker / prioritiser. Ad hoc decision making, and attempting to satisfy everyone, is very stressful and ultimately impossible. Here's a step by step of how to get some clarity, if possible:

1. Slow down.
   
2. Start keeping a record of daily tasks. (Not too much detail. Some ideas: https://davidseah.com/node/the-emergent-task-timer/ or https://qotoqot.com/qbserve/ )
   
3. Once you feel you have a representative sample of the work you do,
   
   1. Categorise the tasks (development, support, mentoring, etc.) and make a pie chart of how much time goes to each.
      
   2. Schedule a 1:1 with your manager, make sure you'll have at least one hour of interrupted time, bring the record with you.
      
   3. Ask them to assign a hierarchy of relative priorities to each of the task categories.
      
   4. Ask them if there are any "priority overrides" (this particular client, or that particular high-level employee.) Each override gets its own category. Who's more important to satisfy immediately, client X or executive Y? It's not your job to make that call.
      
   5. Ask them to divide a pie chart of "weekly work" into percentages of how much they'd ideally like you to spend on each category. Compare that with your existing pie chart.
      
4. On your own, translate the pie chart to hours, given 100% = 40h (or however many it says in your contract.) That's your budget for X category.
   
5. Schedule blocks of uninterrupted dev work, for as many hours as you ended up calculating in point 4. If people ignore them, call the blocks meetings, set yourself as busy, and maybe if your manager agrees, add them as a co-participant in the "meeting".
   
   1. Tip: if you're lucky to work in a place with flexible hours, shift your hours to start earlier than most (and leave earlier), or later than most (and leave later): this will give you a chunk of time when there are few people in the office who are likely to interrupt you.
      
6. Continue keeping a record of daily tasks. Check if your effectiveness goes up. Hopefully it will, so maybe send your manager weekly emails with updates so that they can see prioritisation worked.
   
7. If anyone tries to get you to switch priorities or go over budget on whatever category (meetings, support, whatever), tell them you'd love to, but this conflicts with priorities and goals your manager set out for you and your team. If they try to negotiate, push, or complain, send them to your manager, because the manager is responsible for team resources. Make sure your manager is on board with this before you do so. (But it's their job, so they should be.)
   
   Some notes:
   
   I remember reading a story about Dalai Lama, who recommended meditating for 1 hour a day. When asked what a busy person should do, he recommended meditating for 2 hours a day.
   
   Context switching generates overhead. Your manager may be not experienced enough, or trained enough, to understand that. They may also not be able to visualise the extent of the things you are asked to do, because the requests aren't coming through them. That's what you need the log and pie chart for.
   
   "I don't have the authority to..." is a common negotiation tactic - it works wonders when you're a line employee, but it will backfire if you're trying to increase your authority in the company and aiming for a promotion. In which case you will need to find some other ways to say "no" in a polite way.
   
   All this is designed to take emotions out of decision making as much as possible, while still allowing you to express empathy for the person asking for help. People in need of help are often in distress, and are not thinking rationally, and they will do what they can to resolve that distress ASAP. The task is not necessarily as urgent or important as they portray it - but they will describe it so that you share their feelings and thus their distress, to motivate you to help. So you have to find a way to empathise with them (because people do not take it well when you tell them "you're over-exaggerating") while not letting them make you solely responsible for solving the problem. "I can see it's urgent, and I'd love to help, but my boss..." is one way of doing that.
   
   In general, you have a limited amount of decisions your brain can make every day. Reserve them for the important stuff, and introduce as many routines into your life as possible. There's a number of books about that, for example "How to be a productivity ninja"
   
   (If possible, organise rotating "on call", "tech support" or "bugfix" roles - but this requires buy in from your manager and your team.)