The processing of
your guilt does not diminish even with the act of confessing to
your current beau about your past. The act of forgiveness and
embracement of your past starts with yourself and it is not
something that somebody can give to you.
What I am hearing is that your decision of keeping each other as
a companion is probably akin to lead a non-committed relationship
for six years. This unwise decision, although you have probably
rationalized that you are free to date whoever you want, in
reality, has actually shackle you in some sense. It is indeed a
paradox when it appears that you seem to have freedom, but
subconsciously, you are still somewhat tied on a deeper spiritual
sense (likely a Saturn-influence relationship fueling a karmic
relationship), therefore, how you brand this relationship is not
exactly important (companion, best friend, FB, etc).
Perhaps you are tired in your arrangement with this married man
- merely drifting, with no exact destination to land. Your
experience has taught you that revealing your past will likely to
bring about adverse consequence to your current relationship with
minimal benefits. Hence, I do not see a point for you to go about
relating what has happened in the past when you ought to be
focusing on building a new future with your new beau.
The reason why you failed in your previous relationship is not
so much about you having to share your past with your ex-bf; but
rather, it's about your unnecessary inclusion of him into the
picture of your not-so-glorious past, where there is nothing you or
he could do to alter them. Failing to invest in the present and
look forward will inevitably force you to turn your shoulder over
and quarrel about an illusionary past that produce illusionary
premises for quarrels. And when the false premises are regarded as
real, the death of the relationship will also real - even if it
could be the most promising relationship ever.
You can't have the best of both worlds. If you want to walk
about from your past, you have to sever bonds with that married man
(something which you ought to have done six years ago). If you feel
that you need a companionship to serve as a backup, then you will
always run into the problem of being haunted by issues of the
past.
Start afresh comes with determination and wisdom to steer in the
right direction. To drive faster in the wrong direction will only
make you crash faster... in more fatal manner. 
Cheers
