Typically 'if onlys' would not be used, but we would say, 'what ifs'. Example 1: "Jill, you need to stop thinking about the what ifs." Example 2: "Jill, you need to stop thinking about the if onlys." They can both be used, but example 1 will be found more frequently in English conversation than example 2. :) Hope this helps

In the US, we often talk about accepting how things are, not holding on to our mistakes, working through grief, etc., instead of wishing that things were different, or that one might go back and do things again to correct the negative turn of events. "If only I would have not left that window open, the cat wouldn't have gotten lost." "What if we would have met under different circumstances? Would we have gotten along better?" People often think this way and talk themselves into feeling bad, or not being able to focus on what is going on currently. Although most of us don't talk out loud about "what if's" or "if only's", some do occasionally. There are lots of articles out about stopping this way of thinking in order to live a happier life. The line you mentioned is by a character who found it difficult to stop. Again, this is a "thinking/perception" issue, not necessarily one that people have a lot of conversation about. Hope that was helpful. :)