A couple thoughts-not in specific order regarding your posts with a small amount of history (more can be found in my blog or you are MORE than welcome to PM me anytime).
1. NRE is a reality and you can't make it disappear, but it does wear off. It IS temporary.
I have children. My boyfriend co-parents them with my husband and I. The youngest is biologically my boyfriend's child, but she calls my husband daddy (the truth of her biology is NOT secret).
Never, not one time has there been an issue with having other people take on meaningful roles with my kids.
The oldest child is 20. My ex-girlfriend (dated when child was 2) remains a meaningful part of my daughters life-albeit we lost contact between the two of us, 15 years ago, for 14 years.
My sister is an "alternate mommy" to all of my kids. They call her auntie-but anyone who see's them together can tell that she's MORE than "just an aunt". And-she's not REALLY my sister, she was my best friend in school and it got easier to say "sister" when we were grown instead of explaining because our relationship is clearly more than JUST friends.
2. If you are interested in the OPTION of being a meaningful person in this child's life-just tell the girlfriend. Tell her honestly that right now you are a mess of emotions, with a wedding to plan, counseling to work through, NRE (of theirs) to work through and insecurities-but that you know how precious and important this baby will be in her life and if there is ANYTHING you can do to be supportive and helpful-you are interested in making that effort;
because most of the "issues" you list, are temporary issues that "when the weather passes" won't matter. But, a baby will be around for years.
3. Give yourself a break. You have a lot of stress on your plate. Good or bad, its all still stress-so don't punish yourself for finding it overwhelming.
4. Identify one thing that would be reassuring for you RIGHT NOW. And ask for it. Maybe-as our counselor suggested, give him 3 suggestions and just ask him to pick one and make it happen.
Like... a cuddle night with a romantic movie that he picks-that expresses his feelings for you.
A letter about why/what he loves about you personally.
A cd he makes with songs that make him think of you.
5. This one may be odd to some people, but its worked AMAZINGLY well for me-inquire with her if you could have a "friendly date" with her to go for a walk, hike, bike, shopping trip. NOT TO TALK ABOUT THE SITUATION. Just to be friendly and meet her AS THE REAL PERSON SHE IS, so you can remind yourself that whilst emotions are high-she's still just her.
*having a baby IS a huge thing. But, my daughters father and I had her-and he bailed after 2 months. Completely NOT in love with her or being a daddy.
My boyfriend is madly in love with our daughter but has deep compersion of the amazingly deep, loving relationship she and my husband have-they bonded closer than my husband did with ANY of his own biological children.
That huge thing-could work out completely differently than the way you envision it. So remind yourself-you can NOT predict the future. When you start "envisioning what ifs"-you are trying to predict the future. Get back to NOW-focus on NOW. What is great NOW.
6. Weddings... I haven't met a man who was as "excited" about the PLANNING of a wedding as a woman. I don't know exactly why-but every man I know who is madly in love with their wife and happy to be married-was "so so" when it came to the wedding plans. His reactions to the wedding plans are not necessarily telling regarding his feelings about your RELATIONSHIP.
HUGS. It sucks to be caught up in so much negative emotion and fear.