Hi, I'd like to help you with your question.
It sounds like you have a lot of doubts about both relationships. It is a difficult decision. One relationship offers you stability and someone who you feel comfortable enough to talk to for hours. The other relationship with your ex offers extreme ups and downs and a bit of a challenge, since the two of you butt heads.
You do want to consider that if you and your ex break up a lot, that is an issue to be wary of. There is something fundamentally wrong with the relationship and it would not get better without a lot of work. Although this breaking up and getting back together may be ok now, if you tried to have a future together and possibly children, that type of behavior would not work. You need to find a way to address the problems in the relationship and resolve them before you consider a future together.
The relationship with the nurse sounds like it does not challenge you enough. You indicated that you would not mind if the relationship did not continue. That is a sign that you are either not into the relationship enough yet to see it's potential, or it would just make a great friendship rather than a love relationship.
There is also the issue of trust. You mentioned wanting to be sure that any girlfriend you choose will stay faithful. This could either be an issue with trust with you, or you expect your being away will cause someone to cheat. If the relationship is good and solid and there is love and respect, there is no need to cheat. So looking at it from that perspective may help you. There is never a guarantee with anyone, but building a strong relationship with the qualities above will help you feel more secure in your relationship.
Let me know if I can help in any other way,