To remember the words, you have to use them.
For example, if you read a short article in the language, then try to tell someone what you have read, using the words. Then, after that, try to also write a summary of the same article, keeping in mind the corrections that you received while you tried to tell the person about the article. That way you actually use the words that you learn both in conversation and in writing. You see yourself building sentences and learning to express yourself, rather than just memorizing words. It is hard at first but after a few times it starts to get easier and even fun. This way you see a true improvement in the language instead of just knowledge in the head, and it is very satisfying.
That is what I try to do with my students : )
If you always loose interest, then maybe Spanish is really not the language for you. Maybe you only have extrinsic motivation for learning Spanish, so you thought it would be a good idea to learn Spanish, but you can't develop an emotional bond with this language. It's easier to learn a language, when you're emotionally involved with it and also vocabulary that is meaningful to you sticks much better than vocabulary you just learn, because it happens to be in your textbook, but actually you're not interested in the topic. So, you have two choices: either give up learning Spanish (and why not, if you really aren't enjoying it?) or you try to change the situation and develop a passion for Spanish and try out other learning techniques, that might fit your learning style better with Spanish.
The other users already gave great suggestions, of finding something that might spur your interest for learning Spanish, so I'd just like to give you some ideas, how you could your other languages to make learning Spanish easier.
1. You could read German-Spanish bilingual blogs, like this one: http://www.diariodeunalemol.com/.
2. You could create German-Spanish mnemonics:
arena (= sand): Die Arena ist voller Sand.
matar (= to kill): Der Matador tötet den Stier.
papa (= potato): Papa ist eine Kartoffel.
3. You could listen to German-Spanish music
Tote Hosen "Tagen wie diese": https://www.youtube.com/watch?v=j09hpp3AxIE
Tote Hosen "Días como estos": https://www.youtube.com/watch?v=bRqGZAl7bpY
4. You could watch Spanish movies with German subtitles and vice versa.
5. You could compare German and Spanish vocabulary. both languages share a lot of word derived from Latin (which you are also learning) and Greek:
das Problem = el problema, das Thema = el tema, das Emblem = el emblema
das Museum = el museo, das Spektrum = el espectro
Hello. I think a good way to learn a language is changin your environment. I mean, change all the things that you read,(Phone, messages, news) all the songs that you ear and even you can watch TV in the another language, although knowing that you wont to understand everything at the first moment. Another way is to read books or tales of the spanish language if you like read. Also you must search the gramaticals rules.
Well. Luck learning the spanish.
If memory is your problem, I'll suggest you to use Spaced Repetition Systems (https://en.wikipedia.org/wiki/Spaced_repetition)
Also I found some very interesting tips and tools mentioned in this book: (https://www.amazon.com/Fluent-Forever-Learn-Language-Forget-ebook/dp/B00IBZ405W)