Summary: your employer doesn't care about you. So first and foremost take care of yourself and if you get a better opportunity then take it!
To the story.
I was practically handling HelpDesk alone because my colleagues hated life and couldn't be bothered. To be frank, I never blamed them for this. We all have our issues and mental health is important.
I love my job. I love helping people and resolving IT issues. It makes me glad. So I worked more, took initiative, established rapport with users and became a trustworthy and likeable IT.
Several of my colleagues quit and got at least 10k raise with less workload.
I actually liked my workplace despite the workload. I believe it was due to appreciation and respect I had gotten from users. I didn't want to quit. So I wrote a long mail stating why I should get a raise. I asked for median pay, which equaled about 10k in raise annually.
I got a total rejection. Nada, zero, null.
I almost forgot to mention that a new employee had starting pay about 5k more than me and when I addressed it to my manager before pay raise request, they denied it completely and told me that we are not to discuss wages. Now I couldn't care less and always answered truthfully about pay. Transparency is power to the workers and thus the employers dislike that.
Now I won't lie and I admit that it kinda broke me. I thought that they would try to give me at least a small raise, but no. I was not valuable enough, even though I practically singlehandedly saved a huge project from going under and potentially losing a big client.
I proclaimed to my manager that I will looking for a new job and they had the audacity to say “oh no, not you too”
I still didn't get a raise.
Eventually I got a job offer with 11k raise with less workload and fewer hours, and submitted the notice.
Now to my exit interview. I was honest, yet polite and professional. I didn't want to burn any bridges, because you never know. In retrospect I don't regret it. Polite and professional honesty trumps unprofessionalism in the long run.
Basically I told my manager that I had too many managers, too much of a workload with too little of a pay, colleagues who competed against each and couldn't be bothered to help each likely due to the reasons above.
Just to clarify about managers. I had service manager, technical manager, key account manager, incident manager and of course team manager. At times it was exhausting and frustrating.
I still keep in touch with some of the ex-colleagues and they tell me the situation is gotten worse. The issues are the same mind you, just worse. Which doesn't surprise me as you need the passion I had, to do underpaid and underappreciated work.
My advice is to do well what you are supposed to and not one task more, because you won't be appreciated for the job done. If you get a better offer, then ask your employer to match it or else. If you do extra, then make them appreciate it, cause usually the people above you have no idea what struggle it is to do your job, nor do they care. Make them. Take your sick days, off days, relax, spend time with your family, do what makes you happy and invest time in something or someone who won't break your heart.