REI just isn't for everyone, and if 3 properties are stressing you out then adding 8-12 units will only add to that stress exponentially. You should sit down and really think about whether or not you're willing to put in the hours to make it work, if you should sell and purchase properties that cash flow with a property manager or if you should sell and just reinvest in the stock market. There's nothing wrong with going in any of these directions.
From my experience the 4+ unit buildings are much more demanding than single-family homes and duplexes, I suppose unless the 8-12 unit buildings you're looking at are new. I manage our 27 units by myself, have a full-time job, am full-time in grad school and have plenty of time to spend with my wife and 9 month-old. It can be done, but you either need to fully commit yourself with the understanding that it's going to be difficult and create efficiencies to make things easier or recognize that you should go a different route. The big thing for you is you have time on your side, only being 23. If you can handle the stress in the short term and get the properties stabilized it will benefit you in the long run. But in this specific situation it does sound like the cash flow is too tight and maybe the properties just don't make sense as rentals.
If you do decide to continue self-managing, your lease is your most powerful tool in creating efficiencies. For example, you shouldn't be receiving calls about clogged drains, you shouldn't be cutting the grass or doing snow removal, you shouldn't be physically collecting rent. These items, and much more, should all be addressed in your lease as tenant responsibilities (varies by market), and I'd even include the name/number for your plumber in the event a clogged drain exceeds the tenant's ability to clear so they can schedule directly. Additionally, you need to be proactive with the property's condition or you will find yourself in a never-ending cycle of "minor" repairs which each take up 30 minutes to an hour at a time. I've found that the constant small repair items are a huge drain and the cause a lot of stress. If you can get ahead of it you will save a ton of time and headaches.