My gear box grinds from 2nd to 3rd, 4th to 3rd and 5th to 4th.

So I had a problem with my clutch about a month ago, the clutch got stuck all the way down and I couldn't change gears, this happened while I was on route to my house and because I was about 1 mile away from home, I decided to keep driving it. While driving the last mile home the gears were grinding so as soon as I got my destination I called a recovery truck and sent it to repair. The mechanic had to change the whole clutch and when I got my car back it seems to still grind gears but only the ones mention on the title of this question. Any idea what is the problem? I've being told I need to change my gear box or recondition it but I would like to see how complicated it is as I might be able to do it myself. My car model is a Vauxhall Mokka 2014.

Thanks in advance.