Thats a mechanical problem unfortunately, not F125. It'll be going in to 4th gear and not changing.
If it still drives in limp mode then its likely either the first clutch drum thrust bearing, or the o-ring in the same assembly rather than a broken drum. In terms of gearboxes, thats relatively straightforward to repair since its all just behind the oil pump. Getting the gearbox out is more of an ordeal than fixing it - not a driveway job but something which could be tackled by an advanced DIY-er.
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