I cant stop stressing over this..?

my boyfriend & i broke up about 2 months ago we were together for a year...about two weeks ago i started talking to a guy my sister had wanted me to get to know that she is very close with (her husbands bestfriend).. i didnt tell my ex until about a week of talking to this new guy..and all of a sudden my ex wants me back..before all of this he would say he didnt want to hear anything i had to say that he needed his space because he didnt know what he wanted... i love my ex still but im starting to like this guy ive been talking to a lot. My ex put me through a lot of pain & heartache but all the same time he showed me what love is and we had amazing times together...he says that he is changed and that he wants to give me what i deserve. Im stuck on what to do..this guy im talking to is very nice..my sister is a very hard person to get on good terms with and she likes him. im so stressed out..scared & confused.. does anyone have any advice? ill try to explain more if u need more info