^ being good at guitar does not mean you are good at keeping time. you can play well, but unless you have played with drums or a metronome before, timing is a bit tough. if you get used to playing in free time, forcing yourself into a strict beat is strange at first. if you dont like metronomes cause they sound monotonous (like me) you can use a drum machine to the same effect. play with drum loops to develop your sense of where the beat is and how to stick with it. once you get used to it, you should find it second nature.
buy a drum practice pad and a metronome. it will seem weird to hit a pad to a beat n ur gonna ask urself y u are doing it if ur a guitarist, but it works.

i play drums in a band and our guitarist had the same problem. i made him practice on a drum pad, and problem fixed within days.
While you don't have a metronome, it seems, you can tap your foot to the beat. In common time, 4/4 time, that is, that would be 4 times per measure, matching the beginning of each quarter note.
(I don't mean to insult your intelligence, by any means, but many guitarists haven't formally learned to read/understand music.)
Get a metronome. Start off by tapping along with quarter notes using your foot. Then start playing quarter notes with the guitar. Use a variety of tempos and be sure you can stay locked in with the click for extended periods of time. As you get more comfortable, you can start moving on to 8th notes and then 16th notes. The concepts are simple, but the execution is much more difficult to master.
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