Can someone run a remote software on your computer to control it or get into personal info?

Lately I have been noticing on my history tab that someone gets into my email and views friends requests from Facebook or other sights. I have had instances where I know of a person I had added and they would send me a message (most of them women) and I would not get it, only to be told later by them in person that they have tried to send me a message and the reply would be someone or "me" telling them to either screw off or to stop messaging me. Is there a way to find out who or if someone is getting into my email and doing this? Is there a possibility that someone can be remotely hacking my computer. I have a suspicion that it's my significant other getting into my email and stuff, she is very insecure and very conniving. I also have this suspicion that she could have one of her friends hacking,that is very knowledgeable in doing that or monitoring my email or internet history. It's just my suspicion but now it is getting irritating. This has been going on for years now, which is why I suspect that she is doing this and is lying through her own teeth that she is not behind this. How can i find out? Or how can i figure out if this is what is happening?