HBG, it's all part of the grief process....a process you are barely IN yet. it's normal and it's natural. when you recall the good times, be thankful you had them, but don't try to hold them so tight you lose perspective.
grief counseling would be an excellent idea. when my mom died of alcoholism (i was 32, only child, executor of her estate) i didn't slow down long enough to consider that perhaps i had some stuff to work through. i just soldiered on. that was 25 years ago and it's still a process.
We all wonder WHAT IF sometimes. and not just about the loss of loved one. what IF i'd gone on that trip to France back when i was in cooking school. what IF my mom had allowed my dad visitation, or IF he'd "wanted me enough" to have me live with his family. what IF i'd gone to the true college of my choice, Whitworth, the lovely little liberal arts college nestled in the trees on the eastern side of the state.
makes me think of the movie Sliding Doors, where a woman's life trajectory splits in two. neither is without pain or sadness, neither is perfect, both have flaws and heartache.
allow yourself to feel what you feel. if you are concerned about going too far over the edge, set a timer....allow yourself 30 minutes at a time. then go furiously scrub the grout, or bathe the cat or talk a walk. baby steps.