Am I not meant to be a software developer? The first time I remember wanting to be a software developer was when I was in 8th grade doing a future career project. My first exposure to programming was in high school when I took AP computer science. I did horrible and failed the AP test, but I still wanted to pursue it in college. So I did. I am currently a junior in college with a 2.4 gpa and have done poorly in almost all my CS courses. It takes me much longer to complete programming assignments compared to my peers and I am always last to finish during programming labs. I wouldn't say that I enjoy programming, but I can tolerate it. I think about switching majors at least once a day since freshman year of college, but I could never bring myself to do it. There is nothing else I'm more interested in and there is just something in me that wants to be a successful programmer, but I constantly idealize how much easier it would be if I was doing something else. I am a believer in the notion that you have to be good at something by practicing it over and over before you can enjoy it, not the other way around. However, I am terrified that I will never improve as a programmer. Is the fact that I've done so poorly so far a sign that I should look into studying something else? Any thoughts will be appreciated.
Lots of people (including me) get conflicted about what they want to do with their lives while in college.  So, the first thing I'd like to say is that your uncertainty is quite normal.  But, if you're getting anxious about it, you need to fix that.  

I'd like to suggest that you consider going to an academic counselor at your school.  Find one that you trust to say all of the things that you have said here.  This is just the sort of thing that they are trained for.

I, personally, got my music degree a long time ago.  I didn't do very well at first.  I thought I wanted to be a famous concert pianist, but I really didn't.  I did poorly in school at first, but eventually I decided that I didn't care about being famous, I just wanted to play the piano well and get my degree.  I improved and managed to make it through as a solid 2.5 student. I got to be a good pianist and musician, but not good enough to be a pro. So, a week after I got out of college, I took a job at a ComputerLand store (a big chain back in the old DOS/IBM PC/Apple II days).  I have spent the last 30 years in the IT business, doing one thing or another.  I still practice piano regularly and take students from time to time.

So, a few observations.  If you decide to move to something else, you're not a failure.  If you think that you are interested in nothing but computer programming, you're letting your anxiety get the better of you.  If you "want to be a successful computer programmer" it sounds like maybe being successful is more important to you than being good.  If so, then you need to rethink your priorities.  You won't be happy in any field (or in life, for that matter) if  what others think of you is what defines what you think of yourself.  You need to care about doing a job well, and seek to feel that satisfaction that you get when you do.

I'll suggest again that you get with an academic counselor.  Helping you get in touch with what you really feel like you want to do, and then helping you do it, is their full time job.  Plenty of them are very good at it.  If you decide to switch majors, then you'll get plenty of help.

Above all, don't just sit and do nothing and let your anxiety marinate.  You don't need stomach trouble!  Go do something.  Start examining your desires, abilities and skills, and be tough with yourself about what your feelings and motivations are.  There's nothing wrong with tearing it all down and starting over; you never really do start over because you bring all the life lessons that you learn along with you.  Get to it, and persevere.  :)