This used to be me in spades...though I mostly just kept completely revamping a single comic, until eventually I loathed everything about it and dropped it completely. (Though I started a couple that I dropped very early on too.) I was very, very bad about it and some readers even called me like, the reboot queen. XD
Sometimes though, with new creators especially, you just gotta let them do what they like and figure themselves out - webcomics are a learning experience, after all.
HOWEVER, as someone who used to reboot my comics like crazy, I do try to give tips on how to avoid having too many dead projects lying around -
1: At the beginning, aim shorter. If you're just getting into webcomics, try some experimental one-shots first. It's the comics with no end in sight (or an ending waaayyy too out of sight) that are more likely to be left unfinished. If your projects involve characters or plotlines you REALLY want to use again, you can always leave them open-ended to continue them, or just move on to your grand, epic project after getting the hang of things a bit first. XP It's just better to start out kind of slow and learn your strengths and weaknesses.
2: NEVER start a comic on a sudden inspiration. You want to make an art or some short comic when this happens, cool, but what seems great to you one month may seem kinda lame the next. I mean, sure, we all have moments where we're not as interested in our comics, but if you pick subjects you truly love, it makes it easier to plow through those downer moments. XP
3: If possible, avoid rebooting old chapters because of amateur art/writing. If you absolutely need to for publishing reasons or to change some plot elements, then do what you must, but doing this can REALLY slow things down. That old art/writing may make your eyes bleed, but remember that most readers really don't mind and even enjoy seeing an artist grow.
4: Even with a longer series, have an ending in sight. Again, kind of related to tip 1. Basically, unless you're making a comic strip or slice of life or something, make it possible to reach the end of your comic in your lifetime! Consider trimming unnecessary bulk from your story.
5: Don't let readers' opinions sway you! I don't mean ignore helpful constructive criticism, I mean people who criticize how a character acts, or say "I hope this doesn't happen!" or anything that might be making you second guess parts of your story. I actually spazzed out and changed an entire character because people were considering her too 'Mary Sue', and a lot of the reboots I made were partially influenced by reader opinions as well! Make the story YOU want to make, don't worry about your characters being cliched (heck, even if they are), or people having a different opinion of your characters than you expected. Stay the course! XP