Are you sure this relationship is what is best for you? I'm concerned that it may not be very healthy from a mental health standpoint.
First, she already rejected you once by giving up on you when you were first together instead of trying to work it out. Granted, people can make mistakes and come to realize what they lost. However, you must ask yourself what changed between then and now. Try to recall your relationship objectively. Was it mutually rewarding? Did it validate you as a person, or were there toxic elements?
Second, she is currently dating another guy, and you indicate that she's trying to get back with you. There's a saying, "If they cheat WITH you, they'll cheat ON you."
Remember that there's physical cheating and emotional cheating, so just because you haven't had physical relations recently doesn't mean she hasn't violated the trust of her current partner. If she's truly serious about rekindling your romance, she would break up with her current boyfriend to pursue a possible relationship with you. She'd at least be candid with him that there's some unfinished business with you and acknowledge that she's been talking with you. (Why hasn't she broken up with him already if she's that interested in you? What would the trust be like in your relationship with her if you were to get back together? Very shaky, I'm guessing.)
Third, if she is trying to get back with you, then why do YOU need to do anything to win her back? Seems like she's already plenty motivated. You might want to set some ground rules that you'll only date her when she becomes single -- free and clear. You don't want to sneak around with someone else's girlfriend, do you? Don't you deserve to date someone out in the open rather than share someone else's lover?
First romances often have a very sentimental quality to them in our long-term memories, especially if they were also the first sexual relationship. Don't confuse nostalgia or sexual chemistry for true love, however. Even people who are of middle age do so.