Advice needed, please read all before responding

Hi, my GF and I have been together for 8 years. We recently got engaged 4 months ago and everything was going well. We have had our ups and downs like most couples over the years, but have been living together for 4 years and all seemed great. Over the last few weeks my GF has said to me that she doesnt know if she wants to be together and thinks we are missing something. She does not want to end things yet, as she said we go really well together, she loves me etc. But she has this doubt, that we are missing a spark. From my point of view i can understand to an extend from what she means, but most of my negative feelings are based on knowing she is unhappy. We both want to work on things, but i feel she is doing this as she is scared to move on, I cant get my head around what she is thinking. We cant get passed this stage of worry and concern. We talk regularly about how we feel, sometimes this helps, sometimes we lead to the conversation down the line of what if we broke up, what we would happen. For me this is her trying to easy herself into breaking up, but she said its just a way to try and get past the worry of everything so she can focus on us. For me its been 8 years, ive spent nearly a third of my life with her and I want to marry her, sure we have our ups and downs, but thats part of life, in the end of the day, we have never had a day where she hasnt made me smile or laugh in 8 years? Thoughts are we both putting of breaking up through Fear or is it natural after getting engaged to get anxiety and fear and start to question things?

It is likely that she is scared because marriage is a big step. Although it seems like more than that so although you have wrote that you have both talked and she has told you that she feels that a spark is missing but you need to try and find out what she means by this and whether it might be something that you can fix for exanple if it is sexual then you could try to spice up your sex life or that you aren't spending enough quality time together or if its more than that then it could be that she loves you but is falling out of love with you and if that is the case then it is better to end it before you got more committed because otherwise you would both end up feeling worse. You may also need to make some of the decison to end if she isn't going to. And when you decide where your relationships at then i wish you all the best and if you decide to move on then keep ypurself busy and it will get easier with time. Hope this helps. Lauren