Title: How do i show him that he is not second best?!

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I had these 2 guys who likeed me for a while. i cudnt decide, and ended up finally choosin one guy. as soon as i did, i regreted it, but stayed with him for a couple of months coz found it difficult to dump him.
The other guy, was really hurt, and didnt talk 2 me for a while, but then did agaibn, and carried on really really liking me. He told me he loves me etc, and i love him too.
I dumped my boyf, and me and the guy i love hav been really close at school etc, but hes worrying about being second best, always askin why i chose the other guy over him, 2 which i dont really hav an answer.
i really want 2 be with him properly, but he thinks m gonna hurt him agaijn, which i wont, coz i love him. but how do i showq him this? ive tried tellin him hes not second best, but it doesnt do much good.
He says he wants to take things slow, which im all for, but then he does seem 2 wanna hav a sexual relationship with me, which i only want if we're properly together.
He seems to hav changed, he was never as interested in sex as he is now, hes always with his friends and doesnt wanna spend time with me outside of school, but acts like he really likes me in school. im gettin really confused. and its breakin my heart.
please help!