You are stuck in a dilemma on whether you should
listen to your mind or your heart; regardless, as time pass, you
will eventually discover that you are leading towards the latter.
You should discover by now that by itself and only itself, sense of
responsibility is kinda bullsh|t - you can't retain a person's
feeling through morals and ethics. Love is spiritual, it bypass
through whatever mental barrier you impose on yourself.
You have three choice
(I) to continue your relationship and suffer through being with
someone you have little love with.
(II) To give up present gf and pursue potential relationship
(III) Opt for two timing.
Among all these choices, (III) is totally out of
question IF you want to get out of your mess, because it will
implicate your situation. You can decide about (I) or (II). Lets go
through it slowly then.
What if you stay in the relationship? It is possible as a decision
- however, its not about just remaining and that's it. Attempts has
to be made to rectify the issues in your relationship
(possessiveness, lack of trust, etc) and tons of communication with
your gf. Conclusion, acceptance and lines must be drawn. It is the
'harder' option as expanding effort to REBUILD relationship is
NEVER easy. But it can be very rewarding at the end of the day, if
things work out. Afterall, what makes you think S defintely likes
you the way you do (even though its likely) or things will be
better? And even if it doesn't work out - bottomline is that
you did what you can, instead of running away.

(II) Option? My version of this decision is not to ship-hop at
first sight of opportunity. Probably you already did what you can
in the relationship already; maybe the aftermath of choosing
decision (I) - then if the relationship were to vanish - do not
jump into another boat of love straight away. You may want to clear
up some emotional turmoils and mental issues away first. This will
probably take up to 1 - 3 months. Take this time to get to know S
better as well - don't talk about feelings with
her. Just know as a causal friend. Because if you do, chances are,
you both will be together soon enough, together all these
undissolve matters in yourself and it will bring about greater
problems. 
P.S: Life is about decision making - everyone is doing it everyday.
What matters is that the decision you make, will not bring about
greater issues to settle in the future. You should be rational to
think long term and the possible complication that could occur. Use
all this time now.
Cheers