Unfortunately, the answer you're hoping for does not exist. There is not a happy medium here; there is no situation in which you remain with, and eventually marry, your current girlfriend and get to raise a child who is biologically your own.

The "compromises" you could make (persuading her to have another baby; giving up hope for another baby and making the most out of a step-son relationship; having a baby and taking on the lead parenting role) are all situations in which one of you is making a sacrifice that will make you resentful of one another. You'd be sacrificing something that you need to be happy and live a fulfilling life. She'd be giving up her physical well-being, possibly career opportunities, and the freedoms that come without having a baby/small child (vacations, nights out, sleeping through the night). Either way, it's not right.

The answer is simple but a hard pill to swallow... I think you know what it is: you need to end this relationship. Like a bandaid. It will hurt but prolonging the doomed relationship will only leave you with less time to find and be with the woman you will spend all your years with. The one you will raise kiddos with, and watch them grow up. The one who will be your partner in life, supporting each other through every triumph and trial life is going to throw at you. The heartache you feel about the thought of losing your girlfriend (who you think of as "the love of [your] life") makes it hard to have clarity here. But she isn't the one.

Don't lie to yourself and think you can make do without being a dad. The next year or so will hurt, but it will be worth it when you are 60 years old and having your grandchildren over on Christmas Day.