Sarie, You have a lump in your breast that has grown - this may not be anything malignant, but it is not insignificant. Yes you are young, and that gives you a greater chance that what is there is benign, buts thats all - a greater chance. Your GP cannot diagnose this by touch, neither can the doctor at the breast clinic. They need to give you a triple assessment and a diagnosis to put your mind at rest.
Go back to the GP, see a different doctor if necessary, and tell them how unhappy you are with the situation, how unhappy you were with the last clinic appointment and ask them to refer you for a second opinion. If they balk, take a print out of the information regarding breast lumps on this site, or the one from Coppafeel, which is a charity set up to raise awareness of breast cancer in young women. Syptomatic young women are entitled to have their concerns taken seriously, and your doctor needs to realise that. Ifnecessary take your mum or a friend with you to the appointment, strength in numbers often helps.
By the way, your GP should know that a Clinical Breast Exam involves using the pads of your fingers, not the flat of your hand 😉
Good luck, hope you get some reassurance soon.