I wonder if you're not sleeping deeply because you are hoping to see your mom in your dreams. We have nightmares almost every night, that's how our bodies/minds work out our stress and other issues. Plus what we hear on the news, what is going on around us, etc...we usually don't remember our dreams/nightmares due to the chemicals in the brain that make us sleep.
If you are subconsciously wanting to see her, visit with her one more time, enjoy seeing her then it's possible you are just keeping yourself out of deep REM sleep but are still going through the entire dream cycle.
I suggest you get some over the counter sleep aid medication that you talk about with the pharmacist to make sure what it is, what it might do, if there are any side effects...then take that for the next couple of nights to get your body back on an even sleep pattern.
It is hard losing a parent that a person is close to. It is also pretty normal to actually see them at least once after they are gone. I do believe that the veil between the worlds can be quite thin sometimes.
I saw my dad one time after he was gone, a glimpse at a peaceful wooded setting. I was out hiking at a sightseeing place and turned a corner in the path and there he was, crouching down with one knee on the ground and his arm across the other knee. Then he was gone.
My mom said he came to her night after night for weeks. I was very glad he didn't visit me like that. I would have had a hard time sleeping too.
My friend said her grandmother kept having nightmares night after night after night. She begged my friend to change beds with her for at least one night so she could get some sleep. She said okay and went in, went to bed, and went to sleep. She woke up suddenly and knew he was there. She turned over and he was standing by the bed in his jammies looking at her as if trying to figure out who she was. She looked at him and told him "Grandpa, you need to leave Grandma alone, she needs to sleep at night". He never visited her grandmother again.
So all in all, you need to go fully to sleep so you won't remember your nightmares and get your body back on a regular sleep pattern. If the otc medication is not something you want to try then google Melatonin. Many people take it since it is the natural occurring chemical that normally puts us to sleep. They sell it at health food stores. Most of my friends that use it say they don't feel the effect the first night they take it but if they take it the second night they sleep all night very solidly.