How should I react to my ex telling me she she really misses me and hopes we can get together soon?

my girlfriend and I broke up 5 months ago after about three months of her not knowing what she wanted with end result of her telling me she had started to develop feelings for ex who she had left twice before ,infact she left him for me .there was always a very strong push and pull dynamic coming from her mostly.she is now back with her ex and two days ago she sent a sms saying she really misses me and she hopes we can get together soon. I felt devasted when we broke up and subsequently blocked all contact with her for 4 months .she has tried to make contact but I have not resonded.this was such an amazing relationship and also hell at times.we felt so deeply connected at times and then so distant at other times.towards the end she did some really hurtful things that really broke me down but there was also this loving giving side to her.should I respond .