Should a man give up his life long dream of having a child if the woman he loves finds out she can't have one?

My lady and her 3 year old daughter moved in with me and my two kids now for apporox 2 and a half years, and we all live as a family. (Before she came I raised my two daughters alone as a single father, and proud of it. I enjoy my fatherhood, and want more). However, after years of trying, we have not been able to successfully conceive a child together. Yeah, we are seeking medical help, but its been slow. Why is it imortant? You see, after my break-up with my ex, i've always wanted to start again. To have a lady, for her to settle and share my home, to have kids, to have a family...you know, that perfect vision of kids, a home, a car and a dog...but it doesn't seem to be happening.Its been almost three years now, and I'm now facing up to the fact that for whatever reason we may not be able to have any (we've had the checks, and nothing seems to be wrong with either of us). And the difficulty is, should i bury my want and life-long dream to have a child (she isn't as keen as me, she says she's happy just being with me...but she would have one to make me happy), or should i force myself to just to be content that I have a happy relationship with her? ...and accept that that is my alloted part in life. Or then again should I say to myself, 'we only live once, i want want a child and so whatever means i'll get one'. I know the latter makes me sound really bad, but no matter how much i try to secreatly bury it under the carpet, the desire to start again (with children) is to strong. Quite righty, my lady fears at the end of the day I should be happy with her (with or without kids), ...which I am. She fears, if it is discovered that she can't have any more kids for whatever reason, (remembering she already has a 3 year old daughter from a past relationship), I'll eventually reject her, and either be with someone who can give me a baby, or I'll cheat (and get someone pregnant). All I say is, I don't know how I'll be in the future...in fact, no one can say that they'll be around tomorrow, never mind considering how I'll be with her in the future. But I do admit, I feel a right sod, but i can't help it...and yes, she did know this is what i wanted at the start of the relationship. Can I have some opinions please. I'll be most greatful. And sorry...I don't mean to sound like a crap guy.