You're not giving me enough to go on here, so I'll just shoot from the hip (as usual). Moreover, the statement "I am married and dating someone else..." itself is quite ludicrous (although the situation, I am sure, is not uncommon). However, even in a worst-case scenario, if your marriage is abusive or without joy, I cannot condone adding people to your relationship queue.
You don't tell me why you are looking for a little extramarital something, something. If you are bored and have found someone to whom you are attracted, be aware that it is normal to occasionally have these feelings toward someone other than your spouse. You may have an immature craving to repeatedly fall in love, to experience the giddiness of a blossoming relationship over and over; but these in-love sensations are always fleeting. Relationships evolve and change, and that takes work, but then so does an affair.
And then there is the question of your lover. I recommend that you stop dating this person immediately; what good can come of it? Many times the bloom falls off the rose when the previously hitched becomes...unhitched. Availability dampens the excitement, especially if your lover can no longer escape your baggage. Perhaps you two will fall in love and stay together, but I find this unlikely. There is not much respecting of commitments here. At least one of you will eventually find someone else to pursue.
Absolutely, you are going to have to decide whether you want to be married or not. You need time to think things through, to figure out your wants and needs and whether your marriage and your spouse can provide them, and vice versa. An in-depth conversation with your spouse is in order. Is your marriage so inert that it would not bother you if your partner wanted to take an "on the side relationship further"? As it stands you are merely wasting time.
If your marriage is unhappy and you are unable or unwilling to salvage it then end it. This is not the time to engage in another relationship, except perhaps with a trained therapist.
à tout jamais,