Chances are, you can improve. You're probably a better guitarist than singer. You improved on guitar by playing and practicing countless hours. If you want to improve your singing, you'll need to put in comparable effort. Believe me, my vocal limitations are painfully evident to myself and all around. I've been playing music for four decades and never considered myself a vocalist, not even remotely. But in the past couple of years I've been writing a lot more songs. I decided to work at my vocals, if only to be able to record presentable demos. Fortunately I'm back to working with a good singer, but that would not have happened had I not got to a point where I was able to put out demos that caught the attention of others. Here is what worked for me:
1. Sing every day if possible. Record the results. If you are recording the vocal for a song, try breaking the song into sections. Work on them one at a time. You can start in the beginning, at end, or anywhere in the middle. Pick a section, record (and keep--don't overwrite) eight or ten vocal takes. Then move on. At the end of the day, you can comp together the best bits from your various takes to create a better recording.
2. Figure out which musical keys work and don't work with your voice. Write in, or transpose to, those keys. If your vocal range is quite limited, as most are, it's usually better to pick a key where you are stretching for the occasional high note, rather than one where you are scraping the bottom of your range. Find the range of pitches where your voice conveys the most energy, then choose a key where most of the melody in that range.
3. Don't be ashamed to use a program like Melodyne. If you are using it to improve the final product, with practice you can learn to apply it subtly so that it is all but inaudible. However its most valuable use, I have found, is for practicing vocals. After you record a performance, run it through Melodyne for analysis. The program will give you a graphic view of what you are doing right and wrong. Much better than you can hear on your own. You will find out where you have a tendency to sing flat and where sharp. You'll notice problems like excessive vibrato. Somebody on this forum (was it Chili?) pointed out that identifying these problems and fixing them artificially in Melodyne will improve your ability to sing the song without the aid of Melodyne. I have found that to be so true. Just hearing the corrected version of your vocal will give you, over time, a mental reference that aids your singing. Don't forget, you don't or need to correct every imperfection. Technical imperfection may or may not be musical imperfections. Let your ears judge.
4. Put your work out there for critique. The MP3 Clinic one of the best resources on this site. Don't forget to listen to the songs others have posted and offer comments of your own. In doing so, you are helping to sustain the community while also training your ears to spot problems. Over time, just listening and articulating what you hear will improve your own work.
Hope this helps!