You're not being used, it's just not fair on you! It's a tricky situation because of course, you can't say "I don't want to have to pay all this money to see you", but you can't keep paying all this money into seeing them.
A good idea here is to set a goal. As in, don't just keep taking the flights to see them. Work out a goal with your partner, i.e. you moving in together/living in a country that suits both of you