I wish I had some advice for you, it sounds like you're having a difficult time. All I can suggest is that hormones are so out of control right now and are probably making you feel extremes of everything you might normally feel. Pregnancy is an amazingly complex time for us emotionally.
With my first, we weren't trying, instead we conceived him when I was really ill on the pill and didn't use any back-up protection the next week when he was conceived. It was very tough to get my head round the fact that we were going to be having a baby. We were weeks from the end of term with our teaching jobs after we'd both handed in our notices to go travelling for a year in NZ (travelling around and then settling to work in Wellington). I couldn't believe our dream (we'd been planning it for four years) was over. Those first ten weeks were tough and I had very sombre, dark and lonely thoughts, but then I saw my baby jumping around on the screen at a 10-week ultrasound and instantly the darkness melted away. I still had to let myself adjust to our changed life, but I was filled with a love for my baby that only grew with every ultrasound we had. We still ended up going to NZ, but came back after 5 months in time for the birth.
I think it's understandable that you have mixed emotions; it sounds like it's been a rollercoaster for you since the summer. You say you feel guilt over not being with your previous partner, but if there were good reasons for breaking up with him at the time, then you need to try and remember what they were. I would suggest talking to your new partner about how you're feeling. Maybe he's feeling nervous too and that's what's causing friction between you.
I wish you the best of luck and sorry if I don't have much advice!! Be gentle on yourself.
x
ETA: I forgot to mention that when I was pregnant with my son, my emotions towards my ex changed considerably. We'd had a fractious five-year relationship, which had ended terribly. I had emotionally pulled away from him in the end because he was very emotionally and occasionally physically abusive. I'd hated him for it and even when he sent me very apologetic messages in the years that followed I was still filled with hurt. However, when I was pregnant, I reread those messages and felt a warm feeling for him again. I can't explain why, but being pregnant made me forget the horrible times and made me remember how nice he could occasionally be. I didn't still feel for him, I never could after what he did and the love I feel for my husband could never be broken, especially not by fond memories of a previous partner. However, my feelings did change and it was pregnancy that caused it. Maybe something similar is happening with you?