Check your selector lever, under bonnet, gear selector lever is on left hand side on top of gear box, get your hand in and grab it, if you can wobble it by trying to rotate forwards backwards then the spline is wearing off.
I lost 1st and second gears sat at some lights and had to limp home in 3rd. I took the arm off and the spline teeth had worn away, couldn't tell if the selector shaft was worn too and bought a new arm hoping it wasn't. After bolted it on and all is good again.
New one is solid and I noticed it's ever so slightly thicker walled and is pretty cheap.
Pushing the gear stick forwards and backwards lifts the selector/shaft so that's why I could still get 3rd. Left to right movements rotates the shaft and that's why I couldn't get any other gears.
Hope that helps.