Poland, Czech Republic and Greece are insanely cheap. Don't sign up for tour groups as they'll take you solely into capital cities and push you to buy rubbish. When in France, Spain and Portugal travel by coach as the trains are expensive.
1) Cost up hostels and hotels, when you are absolute on the dates book and pay straight away. Trust me it's a huge benefit.
2) If you limit your daily budget to €30, that covers food, drinks, metro fares and you can buy souvenirs but I recommend limiting to post cards, makes a good decoration at home. Anything money left over can be carried on to the next day. But always have at least €1000 in the bank in case of emergency or if you want to add on extra days.
3) Cook in hostels, you can enjoy local produce. Plus you'll actually enjoy cooking alongside other travellers
4) Interrail is expensive, very! I travelled around Europe in the Summer for two months and used flixbus and local trains. Travel was £280! Interrail passes are almost triple that. Some journeys are long but use long days as a chance to rest or catch up with reading etc... But take trains in the East. I took a 4 hour train in Poland and it set me back €12
Find hostels that have a kitchen, usually it tells you on their description if they do. Cook your own food, have a daily budget of at least €30. If you keep to a budget every now and then treat yourself. Like when I was travelling for two months I maintained a budget but one day a week I would eat out for example. Avoid tourist attractions, meaning don't waste your time visiting expensive and packed museums. Get involved in hostel activities as they are usually free or cheap.
Lastly go to places off the beaten track, easily reached by local train services or cycling