Issue changing our flight

Hello,

we are having an Issue with changing the flight / cancelling the last part of the flight and the customer service agent recommended us to contact you via feedback form.

we have purchased two tickets from buenos aires, argentina to tallinn, estonia via lufthansa website, the booking reservation code is *****.

our plans have changed and instead of having the final destination in tallinn, estonia, we need to arrive to valencia, spain.

we called to customer service and tried to do one of the following things:
a) change the flight route to fly from buenos aires to valencia instead;
b) cancel the last two flights of the current flight -- instead of flying from eze -> fra -> muc -> tll just fly from eze -> fra with the same flight and from there we could book a separate flight on our own to valencia, we saw that lufthansa has a direct flight for that (fra->vlc; lh[números], 05.01.[números]; at 21:00);

we are having the following issues:
1. The only option we were offered when changing the flights to valencia was with a cost of (around) [números] eur! this amount is twice the price the original ticket cost was and is too expensive for us.
2. The agent on the phone told that she was not able to cancel the last 2 flights via her System and there isn't a way to just not take the last two flights and get our luggage in the middle. She also recommended us to write you via the feedback form.

could you please help us with the issue? we are not requesting any money back for the last two flights, we would just like to keep eze -> fra for the same amount that we already paid; or fly directly to vlc.

we really need to arrive to valencia instead of tallinn -- it wouldn't make any sense at all to fly fra -> muc -> tll just to then fly back to frankfurt right away to take the plane to valencia.

is there any way you could help us?

thank you so much in advance!

regards,
ricardo