In need of some serious help with my relationship. Any advice is welcome  

My Fiance and I have been together for three years now, and over the last year we have been having alot of problems. the bulk of the problems started, when we found out that my ex who I have a son by and was wanting to adopt her other baby started to take advantage of me financially when I was paying her support for this kid. She became jealous and angry all the time that I was spending more time with this kid than her own and paying more attention to him. I work offshore and over seas so I am gone for 5 weeks at a time. I only saw my son and the little boy every other weekend when I was home for 5 weeks and I did pay more attention to him because I hardly saw him. Afterward our arguements became much worse when, she started off just saying really mean things to me and then later on throwing things. After that she started pushing me around and then hitting me. I did fire back by saying some mean things of my own as well by telling her to move out and just leave or get her stuff and go. That hurt her alot. At the beginning of 2009, she started playing an online game called final fantasy XI and immediately became addicted to it. I was addicted to myspace games so I know how it goes. She used to argue with me that I was paying more attention to them than I was to her. now all of a sudden the shoe was on the other foot and she wasnt giving me the attention anymore. I went back to work and during this time we argued even more and they got more intense. She has three kids and is more or less a single mom while im away at work. The kids went crazy while I was away and didnt listen to her talked back and fought badly and she couldnt handle it anymore so she sent them to stay with their fathers. It was during this time that she became severely depressed and started to shut me out as well and spent more and more time on the game. I figured I needed to come home from work to fix this ASAP and so I did. I got home and it only made problems even worse because all she kept doing was saying that I was insecure and kept questioning her and this was why she was feeling the way she was because I couldnt stop being insecure. Well after some time of fighting I ended up breaking that game and she got severely pissed at me. I went to stay with family and she stayed home for a week till I came home and then she left. What i didnt know was that she had begun an online relationship with some guy that she had met from her game. a 19 year old kid from another country. I busted her one day when I got into her yahoo email address. afterwards I told her that I would forgive and forget because she hadnt met him in person and obviously slept with him. She did tell him she loved him and even sent out naked photos to him. afterward we tried working out our problems but she was still feeling depressed about everything and needed to get away so she went on a cruise that went into our aniversary on april 21. That pissed me off big time. Now that I am back at work Im constantly feeling insecure and always wanting to question her or look into her emails or myspace or something like that to find something. Am I just being way too insecure? Does she really love me and want to be with me and marry me or is she just biding her time till someone better comes along to take my place and take care of her and the kids financially because she doesnt work?