I’m tired of my name and feel like I don’t identify with it. It seems so foreign when I hear it. I’ve tried on the following nicknames, but none of them feel right. Mer, Miss, [name]Missy[/name], [name]Mo[/name], Momo, [name]Rissa[/name], Missa, Wriss.
I don’t like [name]Mara[/name], [name]Mari[/name], [name]Mary[/name], [name]Maria[/name]. What else is there? :S
[name]How[/name] about [name]Maisie[/name], or [name]Maris[/name], or Massie? Or Misa (mee-suh) or Marsi. Also, I don’t know if you think of [name]Mari[/name] as just like [name]Mary[/name], but you could also use mah-ree. Or you could use a nickname based off of your middle name, depending on what it is.
[name]Issy[/name]? Bit like [name]Izzy[/name] but with a softer sound. Or [name]Em[/name] or [name]Emmy[/name]?
I was in a similar situation as you are in, but I am fortunate to be switching schools for the upcoming year and have the opportunity to unofficially change my name. It’s not all that uncommon for someone to go by a name completely different than their real name. Most assume it is a middle or variation of it. It took my family awhile to accept this (I’ve been complaining for years about how much I wanted to change my name), but I am involving them in the process of choosing my name.
If you are moving soon or going to a new job/school, it is a perfect time to go by a different name. But these opportunities don’t come often, and you have to make sure it’s a name that sticks and you love. And it would be best to ask for guidance from close family members or friends.
If going by a different name isn’t an option, than [name]Maisie[/name] or [name]May[/name] are my favorite nicknames for [name]Marissa[/name].
I like the suggestion of [name]Maisie[/name] or [name]May[/name]. Some others that might work…
Rizzo
[name]Ari[/name]
[name]Sissy[/name]
[name]Mischa[/name]
[name]Maris[/name]
[name]Aris[/name]
[name]Molly[/name]
I like [name]Mari[/name] pronounced mahr-ee, or you could also choose to pronounce it like [name]Mary[/name]. [name]Maris[/name] and [name]Issy[/name] are nice suggestions as well. Also, have you considered using your middle name or a variation of it?
The [name]Maris/nameas I’ve known have been called [name]Mar[/name], or Mariss (muh-riss).
I think Marsi/ [name]Marci[/name] or something, or [name]Maisie[/name], could work.
Honestly though, I’ve never identified with my legal name either and go by something else the majority of the time and I feel much more ‘me’ 
Maybe that’s an option for you? Nothing wrong with going by an unrelated name (it doesn’t have to be a legal name change- just a nickname, if you think it’ll make you happier).