Practice clapping along to music. Start with dance music with a heavy clear 4/4 beat (if you're not sure, ask someone to find you some). Clap along to the bass drum and count to four as you do it, so you feel the first beat of each bar.
Also try clapping to the off-beat -- beats 2 and 4 of each bar. Or pick a drum sound and clap to that even if it's a more complicated pattern.
Dancing - of the organised folk kind - can also be a helpful, fun way of driving a sense of rhythm into yourself, as you're stepping forcefully in time to the beat.
If after all of this, you still don't feel the rhythm, if may be that your brain just doesn't work that way. Some people are colourblind; some people are tone deaf; some people are rhythm-deaf. It's OK, and you can enjoy music in other ways.
Without a sense of rhythm, it will be difficult or impossible to play in an ensemble. But you can perform certain styles of music solo. I have an acquaintance with a beautiful voice and a style of finger picking guitar, who's always asked to perform Rod Stewart's "I Don't Want To Talk About It". His bars are as long as he wants them to be.