Turok I think if you start asserting yourself in a polite manner and following through with your own intentions - it will only feel challenging initially but it will become easier and more comfortable as you move forward.

In other words you can stick to your guns and politely decline these things that your grandparents try to insist on and force on you. Don't take it personal that they are acting in such a manner, and if they feel discouraged or let down as a result of you standing up for yourself, that's only a reflection of them needing to have more realistic expectations of you and to respect your independence more. They very well may get the 'hint' once you start doing this and they'll ease up on their behavior.

So try being more assertive in the face of pushy demands - but do it with maturity, respect, and politeness. Look at it as a game or challenge. Your objective is to preserve your independence and maintain your composure in the face of these 'challenges'.