I sustain all my words together and would like to figure out how fix it.
For example if I sung "I can't wait" (quarter note, quarter, half) I would like to separate 'i' and 'can't' but I always leave them sustained and connected.
Let's make your example a little longer in order to experiment with it, say, "I can't wait, for the day, you will be, here with me", all sung as 2 eighth notes and a fourth.
When I sing it as you describe, with each group of three words joined up, and a pause in-between the groups, it sounds like a standard line from a rock song. However, if I do the opposite, and sing the eighth notes staccato, and stretch the fourth note until it connects to the next eighth note, it reminds me more of a modern R&B song; the line takes on a different feeling, it has more attitude, especially with the stress that the staccato note gives to the "I" at the start of the line.
This little exercise already gives you two ideas to try: One, look outside your genre. Listen to how singers in different genres (and older vs recent music) use staccato or legato to phrase their lines. Practice singing songs from different genres that have interesting phrasing. (This is good advice in general; the most interesting musicians in any genre are the people who also listen to, and take influence from, other genres.)
Secondly, look at how the phrasing (both staccato/legato and dynamics) interacts with the lyrics. "I love you" sounds different from "I. Love. You.", or from "i love YOU"; each version conveys a different meaning. Use this to your advantage when singing, to turn your singing into a meaningful expression of the lyrics. (It's a well-known tip for jazz soloists: whatever your instrument, learn the lyrics to the songs you want to play.)
In the specific instance of "I can't wait", use the "c" of "can't" to separate the words. In order to pronounce the letter C, the tongue briefly halts the flow of air. Extend that moment of haltedness before allowing air through to actually for the C. That is, the back of the tongue blocks the air at the very beginning of a C. Leave the tongue pressed against the roof of the mouth momentarily before allowing air through.
Practice speaking the phrases first. Enunciate each word in a careful, even exaggerated way, being especially conscious of leaving space (more than needed at first) between words.
Practice speaking the phrases on pitch, but not in rhythm. This is the same exercise as #1, except with pitches incorporated. Basically, put a rest between each note.
Practice singing slowly — enough so that the exaggerated spaces between words can be maintained without distorting the rhythm.
Practice singing in time. This will mean shortening the spaces between words, but which should be able to happen naturally now that their conscious presence has been practiced.
Take it slower. Put an extra word between each word - sing 'I-fish-can't-fish-wait'.
Then do the same, THINK the 'fish' but don't sing it. Now, faster...
Think diction. Actually MOVE your lips, mouth etc. to create each sound clearly.
And don't rely on 'getting into the habit'. It's YOUR brain, YOUR body. Take control of what it does.