LittlePebble I've been in a similar situation and have had similar meltdowns:
My mum died when I was young and I'm very close to my cousin (who lives abroad) and lately closer to my aunt. A couple of years ago my aunt said she'd like to be there for me when I have a baby and do whatever she can as I don't have a mum, my cousin also said her mum (my aunt) would be there for me.
I told them both we were trying to conceive (our first) and then told them my ecstatic news that we had a BFP. My aunt was lovely initially then went a bit weird which freaked me out a bit as I have a bit of an 'orphan complex' that people like to pick me up as a project then drop me.
Anyway a couple of weeks later my cousin told me she was pregnant with her 3rd and hadn't told anyone she was trying, she's due about a week before me. I was and am really pleased for her (and loving sharing the process with her) but immediately aware that my aunt would think there'd be a decision to make between who to be with. I had no wish to be the un-chosen one and tbh wasn't sure we'd want her around for the birth etc in reality as she's a bit stressy at times, there then followed a few weeks of my aunt being weird with me until I confronted her and told her she should be with my cousin as she's her daughter and just be happy for me and DP rather than everything being weird.
Despite the honest talk, she's still being strange and going over her reasons for making her decision and I often get distraughtly upset after I talk to her, feeling like I'm opening myself up to being rejected again (all very hormonal and leftover feelings from childhood). DP is very understanding and sympathetic and keeps reminding me that we're going to be a family and will find our way through everything and that there are plenty of friends and his mum who'll be there for us without the mixed feelings.
Sorry to ramble on with my complicated situation but I just wanted to sympathise, I spoke to the MW about it and said I felt that I needed to process, grieve over and accept all my leftover family issues (which I've sorted out 90% of) before baby comes and motherhood begins. She agreed and told me to avoid people who keep upsetting me as I didn't need it. I know it's different as it's your mum but I feel that if I work at putting a boundary between me and my aunt I don't have to get caught in the drama. After all, we're grown ups and about to be mummies and have our own new families, all of which we can share with people who want to share our happy time with us, sod the rest!