It's not that people are saying that you must have a drink or sit back or whatever becase you have to conform, and that they're putting their comfort about yours. It's more than it will just (possibly subconsciously) appear to them that you don't want to be there, and the situation will be uncomfortable and they're not that likely to invite you back. It might not be a deliberate thought, even, that they think you're rude or whatever. It's just going to be an overall impression/feeling that they get from you of discomfort, you don't want to be there, you don't like them, or whatever. But nobody will make you have a drink or sit back or anything, and simply doing those things won't necessarily mean you aren't giving off 'uncomfortable' vibes, either!! It's not accepting a drink that will change things; it's that accepting a drink in a relaxed way that shows you're happy to be there will make the situation more comfortable - or refusing the drink in a polite way that again shows you're happy to be there. The behaviours just are outward signs of what you're feeling, so if you don't want to come across as standoffish, then finding ways to signal that are good (whether it's that you do want to be there but are a bit nervous, or whether you hate being there but don't want to show it, or whatever!). Of course if you do want to send subtle messages, then you have the tools to do so now that you know how the behaviour might be perceived. But no-one is forcing you to do any particular behaviour - it's your choice, knowing how it could be seen. And the more people know you, the better they will get at perceiving your specific behaviours, so if it's clear in other ways that you like spending time with them, then they will be less likely to subconsciously see you perched on the edge of your chair as a signal you don't like being around them!
Maybe you can also change your mindset to thinking that other people often do enjoy catering for someone or making them comfortable as part of hosting, so honestly don't mind being 'put to the trouble' of making a cup or tea or whatever, or letting you sit in their chairs. It might be relief for them too if you show that you're happy to be informal and easy-going. The more you can remind yourself of that, the easier it gets.
Another things to keep in mind is that just because you were brought up a certain way, it doesn't mean you can't change! I had a friend who really seemed to struggle with this, particularly around etiquette type issues. Whenever there was something different that people do - some people think one way is the right way to do something or vice versa, she would always have a "well I was brought up that you always do xxx" or "I was brought up thinking that xxx" or "in my background, xxx is polite" statement, and she couldn't seem to break away from that, despite seeing the evidence around her that everyone doesn't operate that way, that she could change if she wanted to, that as an adult, it's her decision. Even when she could see that it would be easier to change, or maybe even that as an adult, she could see that it wasn't necessarily more rude or polite to do one way, she just couldn't let go of that childhood influence. It was kind of like she was using it as a way of getting out of 'owning' an opinion or decision, somehow - overtly she could say that she could see two sides to something, but "this is how I was brought up thinking" always won out, so that she didn't have to own that she was making that decision or that it might not be the only choice, if you see what I mean. It really started to hold her back in social relationships, I think, because she seemed inflexible and seemed like she was judging other people (when I know she really wasn't) because of this constant "I was brought up to..". Her inability to ever move away from this past came across like she was somehow convinced deep down that her ideas of etiquette were really the only polite way, and other people ended up feeling judged as a result.