Had a very similar situation to you, very dissolutioned at my workplace, constantly over performing only to see colleagus with less experience, worse performance etc get promoted. They'd be happy to recognise my achievements in meetings or newsletters, but always refused to cough up for a training course, delay a request for mentoring or decline my application for an internal promotion, whilst I watched selected others get that after less than a year at the company, despite performances that were measurably atrocious.
I started looking around and quickly realized the predicament. Current job pays above the average, I have some security, and because of the rewarding of others for crap performance, I can easily do what is required and coast on by, taking care of my mortgage and filling up my pension pot each month.
All I had to do was lose my aspirations, and accept the blatant positive discrimination / favouritism that my employer was enforcing under the guise of company policy, accepting I'd never be backed to progress here, that by 40 my career was effectively over because some management group had decided they want a new flavour this time around, and that I'd likely be doing the same job year after year. Sure it's a somewhat enjoyable role, but is it really one I want to do until I retire?
I flagged myself as looking for work and got some headhunters on, attended a few interviews. Was offered three, including one which was basically doing what my current boss does now, plus my current role, at a smaller company who'd value my experience.
So, stay and stagnate in somewhat safety, or go elsewhere, take on more work and responsibilities, discover who I am, but accept the risk? Do they just want what's in my brain, and then I'm disposable? If the money had not been a slight step down, I'd have gone. If I hated my job just a little more, I'd have gone. If we hadn't had a family crisis develop at the same time, I'd have gone. Oh, and I may have had to relocate to the Midlands. But what it did do was give me confidence that I do have the skills and personality to get on in life, even if my current employer will never recognise and , worse, thinks they're covered and have fulfilled their responsibilities to me as an employee because they provide free healthcare for the problems they create in the people they trample.
Now I'm seeing the hopeless promoted lot really mess up the company, so it'll be the push I need to kick on. Cheeky fkers come to me and others , who've been overlooked for years, asking for help to get the out of the crap because they haven't got a clue, almost every desirable figure has nosedived, everyone knows they're already looking at the next jump up and don't care a jot about the current role, and laughably the board just this month announced they are extremely concerned about the loss of community, company identity and togetherness, with some new recruits staying less than a year... wonder why!
There's no easy way to take that choice to go or not, it's human instinct to protect what you have, but if you really think you'll rot in your current role, do something about it, or it can lead to regret and even mental problems. Make some waves, check things out, get more info and see how much each company actually values you. Get some firm offers on the table, until then it's all speculative anyway.