It's not about losing grip on the relationship;
chances are - you have already lost it.
The man is very much a parasite exploiting a woman who has probably
lost much interest in the man she probably only calls as boyfriend
in name. However, the desire is mutual. Her personality kinda aid
the concoction of seduction to his advantage as your woman has the
propensity to indulge in deception of some sorts (afflicted
Neptune).
You are still clinging onto the promise of the past, while she has
already absconded from that era. Using discolored memories of the
past and superficial words that have been said previously are now
cast void and null - taking that into consideration to decide how
you want to steer your future will likely to guarantee you poignant
misery and the fate of having your emotions permanently crushed and
shattered into a million of crystal fragments.
You displayed strong evidence of some avoidance-apt attitude
towards love and relationship. You forcefully deny all blatant sign
of perfidy and chuck it behind your unconscious, thinking that that
would enable you to lead your life 'normally' once again.
The truth is that after your revelation: life will NEVER go back to
normal. There isn't a 'normal' to return to now - everything else
is an aftermath.
Instead of trashing it out on a serious note (probably with a
ultimatum) and if she begs for forgiveness, it must be backed with
actual proof of her course of action - effortlessly, you succumb to
simple feminine tricks and gave in mindlessly.
You think that is love? I tell you that came from
fear.
You have so much fear in your psyche that her one dramatic attempt
at begging for your forgiveness appeared to hold more weight than a
hundred signs of deceitfulness. In your mind, you held back
communication and dialogue because you feared that that would mark
the end of your love, but I can tell you, deep inside you are just
praying that everything will just return back to normal and her
infidelity will just cease miraculously.
Rubbish.
She might be stupid to lie, but you are silly to believe and the
greatest fool to be able to deceive yourself with the most
incomprehensible of reasons.
To catch her red-handed before you will let go?
Perhaps you are just delaying the inevitable: your constant refusal
to accept that this act of promiscuity signified the final days of
your relationship - akin to a dying soldier decapitated by three
gunshot wounds, devoid of medical assistance and merely waiting for
spiritual deliverence to release his soul from pain.
The difference is that your injury is an emotional one.
P.S: When you throw a frog into a pool of water at boiling point,
the frog will jump out instantly. But when you place the frog into
a pool of water and gradually heat the water, the frog will slowly
boil itself to death without escaping. This is the frog theory of
karmic relationship - of both first and third person perspective.
Boundary regression is triggered until a point of maximum stretch
length before it snaps like a wooden ruler - in karmic
relationship, that result often signifies a partial payment of
cosmic debt (CloUdiSm - Yunhaier). 
Cheers
