My girlfriend's family likes to take trips together and my girlfriend always goes with them. The problem is that they often want to include me and I'd rather not go. They cover all costs and are actually really nice about it and I don't want to sound ungrateful, but I simply do not enjoy these vacations. I also don't like the indebted feeling afterwards.
I think her parents' main reason for including me is that they want me and my girlfriend to have a vacation, so one solution might be to plan a vacation ourselves for just the two of us, but we can't quite afford that yet (going on a small vacation doesn't seem to count). Another solution is to come up with an excuse, which sometimes works, but I'm a bit limited, because I don't want to lie and invent obligations. They also sometimes plan way ahead and reserve and pay everything (for me included!) and ask me not to be busy at that time. I can't really decline if it's all already paid.
I'm usually invited via my girlfriend (she acts as the messenger for these things, they're kind of shy of calling me), I need to ask them to invite me personally in the future, but I haven't done so yet.
My parents advise me to go (maybe unrelated, but I rarely go anywhere with my own parents). My girlfriend thinks declining is disrespectful as she holds her parents' will sacred. While on vacation, she also doesn't like to go too far from her family for the same reason - she believes she needs to be close at all times or it's not family time. Her parents don't like taking no for an answer, so it's going to be difficult, but I think I'll just try to ask them not to invite me again, but thank them regardless, while returning from their next vacation.
Any kind of answer is appreciated, even advising me how to convince myself to enjoy the vacations.
To answer the comments
The trips they take mainly consist of going somewhere hot (like Spain) and not doing much of anything. To be honest I can enjoy a similar kind of trip in different company and a shorter duration. The main reason I dislike the vacations is that I find them a bore and a waste of my time (which is not ample). I'd rather spend my free time differently, like going on a smaller vacation with my girlfriend. The secondary reason is that I find her siblings insufferable (sorry if I sound too negative, they'll probably grow out of it, but as it is, they're really spoiled).
We're all Czech. My girlfriend and I live together and we visit both sets of our parents fairly often. We have been together for four years and we're both 24. Her siblings are 19 and 15.