Hi Jonny,
Welcome to the forum
You have discovered the fun of fishkeeping! Think about which fish you like, spend ages trying to narrow your favourites down, make your final decision.....then find out you can't have them because...tanks too small, water is too hard/soft, they can't be kept together because they will mate/fight/eat each other!
Steve raises a very good point, if possible, research the fish you would like first, then get the appropriate tank..too late for you, I know. I had the same problem. My tank was a Christmas pressie so I had no input on size etc, (60L) but had an idea of the fish I wanted to keep. Of course, after using the CC and getting advice from the good folks here, it became clear that I wasn't going to be able to have all the ones I wanted.
I think though, that it makes you choose more carefully and have a better understanding of your tank because you have to know your chosen fish's needs and your tank, water supply limitations. Not much help, I know. Your other option is to do as Sue suggests and treat yourself to one of the beautiful betta fish available, you would also be doing one a huge kindness by rescuing him from the jam jar environment that he undoubtedly has found himself in.
Then, save up.....and get another tank that's a bit/lot bigger depending on your chosen fish's needs!
There does appear to be a well known illness among fishkeepers.....MTS. It stands for multiple tank syndrome....and be warned, it is extremely infectious
You will get any help you need here, everyone on this site is really helpful, knowledgable and friendly
Hope this hiccup doesn't put you off keeping fish, good luck!
Resa