Everything's going great for me at the moment, why can't i let myself just be happy?

after so many bad men in my life i've found a guy who is amazing. He's sweet, y, kind, funny, everything i want and more, and although we have only known each other a short time we get on really well and feel really comfortable together. I am away at uni so it is a long distance relationship but we speak everyday and see each other every 10 days or so for a couple of days at a time. I'm so happy but at the same time i'm so scared that it will all end leaving me feeling even more lonely than before. He has done nothing to suggest that he anything but really likes me but i'm paranoid and i don't want it to ruin things. Every guy i've been with in the past has broken up with me or been nasty to me. I don't want to be cynical and pessimistic at 21! Help.