Hi, Alisha Denicola. The way I see this is you want to have the ability to select one or more project associated for each contact is that correct?
Say you have Person1 and you add Project1 then Project2 so now Person1 has both Project1 & Project2 associated with it.
If this is your situation, then you could change the Control Style of your Projects field to be a checkbox set rather than a drop-down list and use the same Projectsvl value list. This will give you the ability to choose more than one project for each contact. The downside to this is you mentioned that your value list consists of over 100 options. You would not want to display those options as it would take a lot of space on your layout.
The second method is to make your Projectsvl a separate table and relate it to your say Contacts table via a third or junction table (See setting up a many-to-many relationship) and display it as a portal on your Contacts table. This is more complex to set-up but hope give you an idea.
I don't recommend that you store multiple value selections in the same field like you describe for most situations. What you describe sounds like something better done with a portal to a related table where you select each project in a different record of the related table. The resulting set of related records can be much easier to work with than a list of values entered into the same field--such as that produced by selecting check box values or by holding down a modifier key while clicking values in a drop down list or pop up menu.