Please don't beat yourself up. Diet is a minefield which is constantly expanding with new information feeding in, as we are wending our way through and are trying to find a suitable compromise between as natural a diet as possible (basically grass/hay, some wild or dry forage and the odd berry or fruit) and the fact that most owners live in an urban environment and rely on supermarkets.
Pellets were intruced in the middle of the last century to provide a cheaper concentrated form of feed than the natural feed as more and more land was intensively farmed. Unlike what most people think, pellets are not a superior form of feed - and the more pellets you feed, the disproportionally more hay your piggies won't eat.
Please stop fruit; and feed a very little once a week - as a special treat. It is too rich and too high in acid (which can cause potentially fatal lip infection, cheilitis).
Iceberg lettuce is not poisonous and can be safely fed in a pinch. It is however singularly devoid of any nutrients whatsoever and shouldn't part of a regular diet for that reason.
You will hopefully find our diet guide helpful as we have had time to work out the kind of questions that new owners really want the practical and precise answers to.
We have lots more helpful and informative guides on a wide range of issues we get the most questions, worries and cries for help from; including learning to understand guinea pigs, learn what is normal and what not, how to monitor health and weight and how to care for your piggies and also have fun with them safely.
Here is the access link. You may want to bookmark it and use it as useful resource. I am sure that there are some fascinating articles in there which may rather surprise you! The guide format allows us to update our guides as needed; unlike books.
Getting Started - New Owners' Most Helpful Guides
Here is our emergency information access link - another one that you want to bookmark if you are ever in a panic:
Emergency Resources and Critical Illness Care - Contents list and subforum link
Our full and even wider information can be accessed via the Guides shortcut on the top bar but you are of course always welcome to ask any questions you may have. the green links usually give access to a guide which deals with a point in greater depth than we may be able to discuss in a single post; especially as we are all doing this for free in our own free time.
If you please added your country, state/province or UK county to location in your account details (accessed via clicking on your username on the top bar), this makes it appear on the left with every post you make and allows us to tailor any recommendations straight away to what is relevant and available where you are. We have members and enquiries from literally all over the world with different backgrounds, climates, vet and rescue access, differing brand names (including medical) etc... It saves both sides time and allows us to indivdualise our answers instead of giving you the basic advice that fits anywhere. Thank you!