I suggest you practice the two sounds with native speakers and ask their feedback on your pronunciation. Make sure though that you take only advice on this from native speakers of English as learning it from Spanish speaking teachers could confuse you as there are very few Spanish speakers I've met who can really get these sounds down correctly. It's just going to take some serious practice with native speakers or even an accent reduction specialist.

Here's a list of words I just made up that would have the opposing sounds:

Beach and Bitch

Each and Itch

Peach and Pitch

Sheet and Shit

Leech and Lynch

Meet and Mitt

Ditch and Deed

Feet and Fit

Greet and Grit

Heat and Hit

Neet and Knit

Pete and Pit

Reach and Wretch

Seat and Sit

Screech and Stretch and Stitch and Cinch

Wench and Wrench and Witch and Which

Whence and Wince

Have fun! :D

You can hear here the sounds: http://dictionary.reverso.net/ you put the word and you press and put option speak.

1st: The word Bitch can mean:

A female dog, wolf, fox, or otter. 

A spiteful or unpleasant woman.

Was a verb: express displeasure; grumble.

2nd: In word: Bitch you read: Be-tche; in the word Beach you read: Bii-tche

By the way, 'bitch' is not always a bad word - it's a perfectly normal word for a female dog.

As for the pronunciation, the word 'beach' is pronounced with more or less the same long 'ee' sound that you have in Spanish.

'Bitch' has a much shorter sound, and that's the one you need to practice. As the lady in the video says, you need to drop your tongue away from the roof of your mouth, and you have to RELAX your whole mouth.  Then, with your mouth and jaw relaxed, make a very short 'i' sound.

Beach has the sound of bee while the other word has the sound of the bi in big.