Gearboxes hurt ???

hello…. so my omega caravan has original 122 tkm on the clock and since about 2500km the gearbox is getting worse… that makes itself noticeable on the first and very bad in the 2 course….. so if I want to run empty in the first and then want to switch up in the second 1 question… what is that… 2 question… what can or must I change it… 3 question… what does the whole fun cost me… 4 and last question for the first… can I continue driving so you get used to all MFG