How to get over a short fling that turned into heartache?

I'm 19 and was in a long term relationship but we broke up and went our separate ways and I got over it. The relationship lasted 3 years and I learnt a lot from it, I learnt I was far too young to be involved in something so serious and that I should have fun instead so that is what I did. I ended up having a one night stand with someone and we exchanged numbers. I didn't want anything serious it was just fun so I hooked up with him again but I think I've fallen for him. Now he hardly texts me and only texts me when he's been drinking alcohol and wants . I know I should say no but I don't feel that I can, I dont want to be used but he gives me excitement and I really like him but I know he doesn't feel the same even though he says he does. So how do I say no to him? I like him so much it's heartache knowing that he's probably getting with other girls