I remember you some, it's been a while. From what you tell of your life the problem is not what is going on around you (relationship,jobs) the problem is within you. Seems you had at least 2 chances to be happy but that your issues didnt' let you.
Forget about x and what he is doing, I dont' know the whole story but if he's back with her is because he was free to make that choice, you threw in the towel, dont' be angry at him now. You are not a victim, as long as you perceive yourself to be one then there will be always a cloud around you. You are blessed with a new job, and if you were able to get engaged there is no reason why that can't happen again, the thing is you need to work on your issues, mainly letting go of the past and hostility which will erode your soul if you harbor ill thoughts about x, he isnt' the main problem.
Being optimistic is an everyday task, is convincing yourself that you'll do finel...eventually your mind will catch up. Soul search and forgive, I have just realized how harmful it was for me to remember every indignity x put me through when he was cheating on me, remembering just made me angry and bitter, I was teethering myself to him, and that's the last thing I want.
Be not afraid...I will repay you for the years the locusts have eaten Joel2
30something 2kids survivor of S, MLC, A, D I have peace in my heart, at last.