There are a few things to take in account when closing a letter to a loved one. So, let's get started on what you would need in order to get the closing statement closest to what you desire to express.
First, the use of "tuyo" versus "suyo" which has previously been discussed in this question. In short, the difference in this case would be of familiarity: Suyo is more respectful and formal, while tuyo is more familiar.
Usually, when you address a loved one you do it in an informal manner, which would tip the scales in favor of using "tuyo"; however, there are cases where you still address a loved one (such as a parent or a grandparent) with respect, in which case it would be "suyo".
Keep in mind that some spouses refer to their partners respectfully, specially in cultures where arranged marriages are a norm. So, as a rule of thumb, if you would refer to your loved one as "sir" or "madam" use "suyo", if you wouldn't, use "tuyo".
This takes us to the second aspect to address: After deciding to use either "suyo" or "tuyo", you might need to modify it to match your gender. If you are a male, you don't need to make any modifications, if you are a female, you would use "suya" or "tuya".
The final part would be what you intend to convey with your closing statement. "Yours" would be simple, where you either use "suyo/a" or "tuyo/a", while "Only yours" might imply "Exclusively yours".
If your intention is to express "Only yours", the easiest way to translate it would be: "Sólo suyo/a" or "Sólo tuyo/a". This phrase would give you the most literal translation possible.
However, if you want to imply "Exclusively yours" or you want to be a little more creative, here are a few choices:
- Solamente (s/t)uy(o/a) (Only yours)
- Exclusivamente (s/t)uy(o/a) (Exclusively yours)
- Únicamente (s/t)uy(o/a) (Uniquely yours)
- Enteramente (s/t)uy(o/a) (Entirely yours)
- Completamente (s/t)uy(o/a) (Completely yours)
- Absolutamente (s/t)uy(o/a) (Absolutely yours)
- Indudablemente (s/t)uy(o/a) (Undoubtedly yours)
- Apasionadamente (s/t)uy(o/a) (Passionately yours)
- Eternamente (s/t)uy(o/a) (Eternally yours)
- Sincereamente (s/t)uy(o/a) (Sincerely yours)
For example, if you were a female and informal, you would use "Solamente tuya" but if you were a male and formal you would use "Solamente suyo".