we may grammatically put it this way:
I think too much--我想太多
我 想 太 多
I think very much-我想很多(it's a very odd expression in oral Mandarin, but here what I want to show you is the difference between too much and very much) 我 想 很/非常 多
alright, these are not important, I think your question focus on "think", simply say, 我思很多, "思" is incorrect here. It's normally a classical Chinese verb when we express something relate to "mind activity" like "think-思考" or "miss-思念". there is a very famous classical poem by great poet Li Bai, 低头思故乡, refers to "missing".
In a word, it's better to say 我想太多 for "think too much", if we add a word "了" to the end by 我想太多了, it would be much more comfortable in every day spoken Mandarin.