I've been going out with her for 4 months now, we make each other happy and like spending time with each other. She met her ex at uni like 8 years ago, got engaged to him after three months, then moved down here with him 4 years ago. Their relationship ended two years ago (not sure on what terms and this is just a rough timescale as we've not spoken directly about it). She still lives in the house they lived in when they moved here. She mentions him (not by his name but as her ex) a bit more often than I would like but I get it because they were going out for 6 years so a lot of her experiences will be with him. She's already said to me (half jokingly) about us maybe getting married. She still has the dress that she didn't get to wear for the other wedding. She seems to be slotting me into the space that he filled. We met on a dating website. Last night when I was closing up at work she was sitting in the bar, one of the other barman's friends were in the bar. Turns out she went round to a bbq at his house last year. I asked how they knew each other and she muttered something about her ex, but I didn't catch it properly. I don't know what to do or even what I hope to achieve by posting this, maybe just to get some advice.

You could always tuck it away for future references when you have clear facts by which to base any opinion on. Why make it a big deal when you yourself have only been going out with this person for 4 months, and have MUCH more to learn.

4 months is too soon to panic in MOST cases, or even commit fully to a stranger. Be cool, and enjoy the learning more. If you can't, its time to go on to better things. Wonder what YOUR past looks like?

In any event, don't let her repeat the quick marriage plans with you.

Dating is normally a step into a long term relationship and marriage

If you have no plans on a serious relationship with this girl, then just keep it there but let her know.

In a way, a new boyfriend or girlfriend is always a replacement of the old, since we are replacing them.

Long relationships can have a long hold on a person, however, because of the period between that one and the one you and her started I would say she isn't using you as a rebound if that's what you are saying. Maybe that was her first real relationship and she has nothing else to compare her current situation with? Tell her, that although you are very interested in getting to know her, that you accept her past with her ex and to cool it on mentioning him so much. However there is nothing alarming unless you find her withdrawing away. Also, this has happened to me twice in a row recently, where one of the girls I am dating had a fiancée who passed away whether for natural or unnatural causes... this seems to hold a VERY deep wound, one which you might have to help her cope with. Maybe finding out about her past isn't too bad, after all, you are interested in getting to know her right?