i think at this point u've been thru so much that u need to stop worrying abt everyone else and jus take it easy for a while. i dunno- go away for a week n relax, hav fun !! but also stop thinking abt hurting other ppl's feelings. that'll get u nowhere. u need to find out wht YOU want.
but u do admit that u only went to the second guy haflheartedly and that u werent completely 'with' him. maybe u used him as a rebound guy without really meaning to.
i think u need to be honest with him, otherwise its gonna come as a huge shock if u do decide to go back to ur ex, and it mite make him more depressed.
if u go back to ur ex, dnt think only of the gud times, try n think y it was in da first place that u 2 broke up...the last thing u need is anotha replay.