There have been a few threads on here about runaway partners (usually men) and the anguished abandoned partners (usually women) with a sort of common theme of the abandoned partner having 'tried so hard' to keep the other one in the relationship/move the relationship on to the next stage etc. IF someone doesn't want to be in a relationship with you, or to be in a monogamous relationship with you, then you might be able to cling on to them for a while by a mixture of tactics including bullying, whining, coaxing and trying to fulfil the person's every whim - but they will still take off in the end because they are NOt That INto You.
So irf you are seeing someone who says any of the following -
'I'm not sure if I love you'
'I'm not good at being faithful'
'WHy do things need to change, we are happy as we are (when it's a matter of moving intogether, getting married or TTC)'
I need a lot of space
Then try believing this person. He/she either doesn't want a committed exclusive relationship at all, or doesn;t want one with you - even if all your friends and family say the relationship is wonderful. He/She will go eventually, often with far more cruelty than necessary, simply because he/she resents the pressure you applied.
There are plenty of available prospective partners who do want exclusive committed long term relationships, so why waste time trying to enforce commitment on people who don't want it?