What's wrong with my peppermint plant? :(

I started growing this peppermint plant from a cutting around July of last year (2017), and after propagating I moved it into this pot with well draining soil & drainage holes, watered it regularly and it grew to the size shown below, however for the past six months it's been exactly this size!
No matter what I do it won't grow any further, I followed the proper water and sun directions for it (low/indirect sun) and when that didn't work I put it out in more sunlight (this was around 2 months ago now), which helped one new sprout pop up, but I just checked on it this morning (this photo was taken last week) and the main leaves are starting to brown The new leaves that just grew in look mostly okay(ish) though.
Is my plant dying or can I fix it? I don't really want to start over again if I can prevent it as I feel like this little thing is my baby and I want to save it if possible, but I'm not sure if I can or what the problem even is!
Any and all advice appreciated, thank you!
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Mint dies down in the winter. It should reappear in the spring. Keep it in a sheltered spot outside.
Gardening in Central Norfolk on improved gritty moraine over chalk ... free-draining.
Thanks dove! Do you know by any chance why it hasn't grown in the past 6 months? It's been stuck at the same size since it sprouted.
It's probably making roots. It should resprout in Spring, earlier if kept in a sheltered position up against a wall.