I've lost him... and half of myself, how can i prove myself to him? PLEASE HELP?

It's a bit long but bare with me, this is very important to me. We were the perfect couple, i won't go into detail because i'll forget to stop and we'll be here forever, we were together 9 months grand total, we were really close, never argued once, match made in heaven, i messed things up one drunken night, went behind his back, he probably would have forgiven me but i didn't tell him, things escalated, we've been split up for about 5 months now, but we're good friends. if i had another chance with him i'd never ever take it for granted, i need to show him that i love him and that he can trust me but i don't think he even knows i have feelings for him, how do i go about doing this? he's irreplaceable and life without him is miserable, i need him so that i can be me... please please help