It might be a good idea to keep a little notebook of words you cannot spell. You can set aside a time to find those words in a dictionary and write the correct spelling in your notebook.
Spelling, especially English, is tricky. It is not rigid and many words can be spelled more than one way. Don't be too hard on yourself. With practice, anyone can become a good speller.
You know alphabetical order, so that is a big tool for finding words. Say you want to learn how to spell "separate". You know it begins with an "s" and it is followed by a vowel. You start in the dictionary at the letter "s" and choose the vowel you think is most likely, like "e" so you are looking under "se". You sound out the word and figure out that the next likely letter is "p" so you are now looking under "sep". You will find "separate" pretty quickly as you read down the list of words in the dictionary.
The problem is harder when you do not even know the first letter of the word. You will have to choose somewhere to start for a word like "catamaran" you would start either in "c" or "k" and follow the same procedure by sounding out and trying to figure out the next letter or letters.
You will learn a great deal about spelling by doing this and though it may seem tedious, it is a lot better to go through an exercise like this and learn a correct spelling. It is equally important to read the entry's definitions when you find the correct spelling of the word. Reading the entry reinforces your learning and sets that word in your memory. Remember that correct spelling is key to conveying what you mean in writing.
Sometimes a word is in a dictionary as an inflected form of an entry word or as a derivative or run-on. By learning about all the features of your dictionary, you now know to check all parts of an entry and will often find spellings in other parts of a dictionary entry than the headword itself.