Our principle cornet player tried to replace his third valve trigger spring. After a couple of hours of swearing at it and crawling round on his hands and knees to find the spring after it had flown off he took it back to the shop and they put it on in about 5 minutes.
So my advice is to swallow your pride and take it to a music shop! :lol:
They are an absolute pain to fit! You have to "pre-compress" them just to get them in the correct position. I managed it by fitting it over the lever, then pushing it down so the two ends go over the back of the holder bit, then pushing it in and very quickly inserting the little bolt. Took 3 or 4 tries, though.