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If I want to say in Latin I speak Latin easily, I say:
In Latīnā facile loquere possum.
But if I want to add to this the idea that it is in my opinion that I speak Latin easily, do I simply use the dative of reference, like this:
In Latīnā tibi facile loquere possum?
Let me start by fixing your initial sentence. My take on translation:
I can speak Latin easily.
Latine facile loqui possum.
Important points:
Next, I'll offer a completely different alternative. It is common to express such things with the supine ablative. My suggestion with an overly literal English translation:
Lingua Latina (mihi) facilis est locutu.
Latin language is easy (for me) with respect to speaking.
As brianpck suggested in a comment, this is another good option:
Facile est (mihi) Latine loqui.
It is easy (for me) to speak Latin.
Finally, let me answer your question. In English you could add a parenthetical "I think", like this:
Latin is easy to me, I think.
In Latin, a similar remark could be worded as credo (I believe) or puto (I think). The ut opinor suggested by Hugh is also a good option, and can be used similarly.
(Also, tibi is second person, meaning roughly "to you". Perhaps you meant mihi, but that would be weird, too.)
There is a ready-made phrase, "ut opinor," which, in my opinion, avoids a lot of ambiguities and other problems.
Suggero 'ut opinor,' quamvis non examussim, ut opinor, tibi respondeo,
I suggest ut opinor, although I am not, in my opinion, answering you precisely.
If looking for a word meaning 'fluently' we have the authority of Cicero (Brutus, passim) himself. Keeping things simple, I suggest:
lingua latina solute utor, and
credo me lingua latina solute uti.
