I guess you're not rooted.,correct? Correct
Are you on sprint? No, Verizon
Does this occur anywhere in your area or certain areas? Anywhere in my area
Are you on WiFi when this happens? If you turn off wifi and test do you still have the same issue? I am on WiFi when this happens, but also when I'm out of range of WiFi in a 4G
What messenger are you using on your device? Stock or 3rd party? Stock
Are you using Google hangouts? Google voice? Any other voip app? No to all of these
Did you soft reset? Yes, often, and that usually breaks the currently held-up SMS message, but it won't stop it from happening again shortly.
What carriers are you sending texts to? Does this happen no matter what carrier you send to? Verizon and Sprint are the two I know I'm sending to, and they are affected equally.
What happens when people send you a text? Do you receive them on time? Are they delayed getting to you? If I don't currently have a hung-up SMS, I get texts from others immediately. If I have do currently have a hung-up text, once that text finally sends, I will get any outstanding incoming texts all at once immediately thereafter.
Can you access data (say can you stream youtube or access the Internet from your browser)? Yes, data is no problem.
Did this just start happening or has it been happening for a while? It's been happening for at least 6 months.
Have you contacted your carrier to see if there are any temporary cell outages in your area? No, and the problem seems longer that temporary at this point.
Answering these questions will help more than you know...