How do you deal with someone coming back into your life?

I was really into this dude about two years ago and he just left without a word. I couldn’t describe how wonderful he made me feel. For the longest time, I was constantly thinking of how I would find him again. Evidently, I found him. He’s been texting me and now he’s doing everything like how it used to be. But, I feel all my feelings are fading for him. How do I deal with this? I’m so confused on my feelings. I feel like I should just let him go, but I know deep down, he’s one of the people that made me the happiest I’ve ever been. I’m not sure if I was just someone he needed to kill time with. Everything he said to me SOUNDED illegitimately, honest. But then again, I could be wrong.