First off, Congratulations on your pregnancy! I'm sorry you are dealing with morning sickness. That makes everything so much harder. I think you may be right when you said that the first trimester is influencing your feelings. I'm thinking that once you are out of that icky feeling, and showing, and feeling baby move and all the good things about the second trimester your outlook may change.
Second, Maybe reading some motivating books could help. Some good ones off the top of my head that mostly portray birth and pregnancy in a positive tone are A Good Birth, A Safe Birth, The Sears Pregnancy Book, and Spiritual Midwifery. Also, Birthing From Within, and another book that Ina Mae Gaskin wrote.... just last year I forget the name but it had so many enlightening stories in it. anyone remember the title?
Third, Talk to your doula. That's what she's there for! I never push my clients into a natural birth- their birth is just that- theirs, not mine. Certainly there are numerous health advantages to a natural birth, and I know that you know that. But you are the one feeling what you're feeling. So talk to her. Maybe she can help you navigate through what you are feeling. Having a doula doesn't mean you HAVE to have a natural birth, kwim?
Fourth, Talk to your midwife. She too should be able to add some insight as to what you are feeling.
Are you under other stresses too? Taking care of two little ones and being pregnant is a big job. Maybe taking some time to yourself and doing something pampering for you can help relieve stress.
I'm sorry you're feeling this way. Please let us know how it's going, ok?